diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4d9a4ce9..19bca2ef 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -130,6 +130,9 @@
 - 쪽 수 추정(실험, `estimate_pages`)이 탭 정의를 한 번만 읽는다. 전에는 글자 모양을 볼 때마다 머리 정보의 탭 정의 전체를
   다시 읽어 큰 문서(185쪽)에서 6분 가까이 걸렸다(이제 약 2분). `write_line_caches`는 쪽 수 추정이 따르지 못하는 첫 구역에서
   멈춘다(그런 문서에는 어차피 아무것도 쓰지 않는다). 그 구역 하나의 사유만 알린다.
+- `doc.styles.replace_font`가 바뀐 글꼴을 쓰는 문단의 줄 배치 정보(`hp:linesegarray`)를 지운다. 한/글은 남은 정보의
+  줄 나눔을 그대로 따르므로, 전에는 새 글꼴이 옛 글꼴의 줄 폭에 맞춰 눌리거나 겹쳐 보일 수 있었다.
+  `doc.parts.set_paragraph_auto_spacing`도 값이 바뀌면 그 문단 모양을 쓰는 문단의 줄 배치 정보를 지운다.
 - FormFit과 쪽 수 추정이 한양 글꼴(한양신명조·한양중고딕·한양견고딕)의 굵은 글자를 em의 1/20만큼 넓게 잡는다. 한/글은
   그 폭을 더한 뒤 배치 단위로 반올림한다(14pt "가"가 1,400이 아니라 1,472). 전에는 굵은 글자도 보통 폭으로 셌다. 다른
   글꼴의 굵은 글자는 전처럼 보통 폭이다. `TextStyle.bold`가 글자 모양의 굵게를 담는다.
diff --git a/src/hwpx/oxml/header_compat.py b/src/hwpx/oxml/header_compat.py
index 97058ecb..c2caba27 100644
--- a/src/hwpx/oxml/header_compat.py
+++ b/src/hwpx/oxml/header_compat.py
@@ -304,6 +304,7 @@ def apply_paragraph_auto_spacing(
         )
         auto_spacing_elements = [auto_spacing]
 
+    before = [dict(auto_spacing.attrib) for auto_spacing in auto_spacing_elements]
     for auto_spacing in auto_spacing_elements:
         if e_asian_eng is not None:
             auto_spacing.set("eAsianEng", _zero_one_bool_str(e_asian_eng))
@@ -315,3 +316,8 @@ def apply_paragraph_auto_spacing(
             auto_spacing.set("eAsianNum", "0")
 
     header.mark_dirty()
+    document = header.document
+    if document is not None and before != [dict(a.attrib) for a in auto_spacing_elements]:
+        # the gaps between Hangul and Latin or digits change, so do the cached line breaks
+        from .header_fonts import clear_line_caches
+        clear_line_caches(document, para_pr_ids={para_pr.get("id")})
diff --git a/src/hwpx/oxml/header_fonts.py b/src/hwpx/oxml/header_fonts.py
index 038e8423..2c689332 100644
--- a/src/hwpx/oxml/header_fonts.py
+++ b/src/hwpx/oxml/header_fonts.py
@@ -35,6 +35,8 @@
     _FONT_FACE_LANG_TO_REF,
     _HC,
     _HH,
+    _HP,
+    _clear_paragraph_layout_cache,
     _element_local_name,
     _normalize_font_langs,
     _validate_font_type,
@@ -42,9 +44,10 @@
 from .header import Font, parse_font
 
 if TYPE_CHECKING:
+    from .document_parts import HwpxOxmlDocument
     from .header_part import HwpxOxmlHeader
 
-__all__ = ["border_fill_info", "font_face", "fonts", "replace_font"]
+__all__ = ["border_fill_info", "clear_line_caches", "font_face", "fonts", "replace_font"]
 
 #: ``hh:borderFill`` side children, in ``BorderFillInfo`` field order.
 _BORDER_SIDES = ("leftBorder", "rightBorder", "topBorder", "bottomBorder", "diagonal")
@@ -226,6 +229,11 @@ def replace_font(
     *dst_face* it is ``<hh:font id=… face=dst type="TTF" isEmbedded="0"/>``.
     *font_type* (``REP``/``TTF``/``HFT``), when given, sets the ``type`` of
     every font this call appends; fonts that already exist are not changed.
+
+    Every paragraph with a run whose ``hh:charPr`` now names another face
+    loses its line layout cache (``hp:linesegarray``): the cached line
+    starts and widths belong to the old face, and Hancom keeps a cache's
+    line breaks, so the new face would be drawn squeezed into the old lines.
     """
 
     src = (src_face or "").strip()
@@ -259,6 +267,7 @@ def replace_font(
         None,
     )
 
+    faces_before = _char_faces(header)
     changed: list[str] = []
     declared: list[str] = []
     repointed = 0
@@ -323,6 +332,11 @@ def replace_font(
         document = header.document
         if document is not None:
             document.invalidate_char_property_cache()
+            faces_after = _char_faces(header)
+            clear_line_caches(
+                document,
+                char_pr_ids={char_id for char_id, faces in faces_after.items() if faces_before.get(char_id) != faces},
+            )
     return FontReplaceReport(
         src_face=src,
         dst_face=dst,
@@ -330,3 +344,50 @@ def replace_font(
         repointed=repointed,
         declared=tuple(declared),
     )
+
+
+def _char_faces(header: "HwpxOxmlHeader | None") -> dict[str | None, tuple[str | None, ...]]:
+    """The faces every ``hh:charPr`` names, one per ``hh:fontRef`` language, by charPr id."""
+
+    if header is None:
+        return {}
+    faces: dict[str | None, tuple[str | None, ...]] = {}
+    blocks = {lang: _fontface_block(header, lang) for lang in _FONT_FACE_LANG_TO_REF}
+    by_id = {
+        lang: {font.get("id"): font.get("face") for font in block.findall(f"{_HH}font")}
+        for lang, block in blocks.items()
+        if block is not None
+    }
+    for char_pr in header.element.iter(f"{_HH}charPr"):
+        font_ref = char_pr.find(f"{_HH}fontRef")
+        faces[char_pr.get("id")] = tuple(
+            None if font_ref is None else by_id.get(lang, {}).get(font_ref.get(attr))
+            for lang, attr in _FONT_FACE_LANG_TO_REF.items()
+        )
+    return faces
+
+
+def clear_line_caches(
+    document: "HwpxOxmlDocument",
+    *,
+    char_pr_ids: Iterable[str | None] = (),
+    para_pr_ids: Iterable[str | None] = (),
+) -> None:
+    """Drop the line caches (``hp:linesegarray``) of every section paragraph whose
+    ``paraPrIDRef`` is in *para_pr_ids* or that has a run whose ``charPrIDRef`` is
+    in *char_pr_ids*: an edit changed that shape in place, so the cached line
+    breaks no longer fit, and Hancom keeps a cache's line breaks.
+    """
+
+    char_ids, para_ids = set(char_pr_ids), set(para_pr_ids)
+    if not char_ids and not para_ids:
+        return
+    for section in document.sections:
+        removed = 0
+        for paragraph in section.element.iter(f"{_HP}p"):
+            if paragraph.get("paraPrIDRef") in para_ids or any(
+                run.get("charPrIDRef") in char_ids for run in paragraph.findall(f"{_HP}run")
+            ):
+                removed += _clear_paragraph_layout_cache(paragraph)
+        if removed:
+            section.mark_dirty()
diff --git a/tests/test_header_compat_authoring.py b/tests/test_header_compat_authoring.py
index f736a98c..016cbf17 100644
--- a/tests/test_header_compat_authoring.py
+++ b/tests/test_header_compat_authoring.py
@@ -291,3 +291,29 @@ def test_real_corpus_document_options_round_trip_untouched(sample_path: Path) ->
         else []
     )
     assert [p.auto_spacing for p in props_before] == [p.auto_spacing for p in props_after]
+
+
+def test_set_paragraph_auto_spacing_drops_the_line_caches_of_its_paragraphs() -> None:
+    """The Hangul-Latin gaps change, so the cached line breaks of the paragraphs
+    with that shape no longer fit; other paragraphs keep their caches, and a
+    call that changes nothing keeps them all."""
+
+    from lxml import etree
+
+    hp = "{http://www.hancom.co.kr/hwpml/2011/paragraph}"
+    document = HwpxDocument.new()
+    para_pr_id = document.oxml.headers[0].ensure_paragraph_format(alignment="LEFT")
+    edited = document.add_paragraph("한글 English 123").element
+    edited.set("paraPrIDRef", para_pr_id)
+    kept = document.add_paragraph("다른 문단").element
+    kept.set("paraPrIDRef", "0")
+    for paragraph in (edited, kept):
+        cache = etree.SubElement(paragraph, f"{hp}linesegarray")
+        etree.SubElement(cache, f"{hp}lineseg", {"textpos": "0", "vertsize": "1000"})
+
+    document.parts.set_paragraph_auto_spacing(para_pr_id, e_asian_eng=False, e_asian_num=False)
+    assert edited.find(f"{hp}linesegarray") is not None  # already 0/0: nothing changed
+    document.parts.set_paragraph_auto_spacing(para_pr_id, e_asian_eng=True)
+
+    assert edited.find(f"{hp}linesegarray") is None
+    assert kept.find(f"{hp}linesegarray") is not None
diff --git a/tests/test_style_font_border_reads.py b/tests/test_style_font_border_reads.py
index e7f1e797..6a71c8fa 100644
--- a/tests/test_style_font_border_reads.py
+++ b/tests/test_style_font_border_reads.py
@@ -777,3 +777,54 @@ def test_replace_font_rejects_an_unknown_font_type_before_mutating() -> None:
 
     assert caught.value.code == "style-font-type-invalid"
     assert etree.tostring(_header(doc)) == header_before
+
+
+# ---------------------------------------------------------------------------
+# replace_font and the paragraphs' line layout caches
+
+HP = "{http://www.hancom.co.kr/hwpml/2011/paragraph}"
+
+
+def _cached_paragraph(doc: HwpxDocument, text: str, char_pr_id: str) -> etree._Element:
+    paragraph = doc.add_paragraph(text).element
+    for run in paragraph.findall(f"{HP}run"):
+        run.set("charPrIDRef", char_pr_id)
+    cache = etree.SubElement(paragraph, f"{HP}linesegarray")
+    etree.SubElement(cache, f"{HP}lineseg", {
+        "textpos": "0", "vertpos": "0", "vertsize": "1000", "textheight": "1000",
+        "baseline": "850", "spacing": "600", "horzpos": "0", "horzsize": "42520", "flags": "393216",
+    })
+    return paragraph
+
+
+def _texts_with_cache(doc: HwpxDocument) -> list[str]:
+    return [
+        "".join(t.text or "" for t in p.iter(f"{HP}t"))
+        for section in doc.oxml.sections
+        for p in section.element.iter(f"{HP}p")
+        if p.find(f"{HP}linesegarray") is not None
+    ]
+
+
+def test_replace_font_drops_the_line_caches_of_paragraphs_it_changes() -> None:
+    doc = _decor_doc()
+    _cached_paragraph(doc, "바뀌는 글꼴의 문단", "0")
+    _cached_paragraph(doc, "그대로인 글꼴의 문단", "1")
+
+    doc.styles.replace_font(DECOR, DST)
+
+    # the old face's line breaks would squeeze the new face; the other paragraph keeps its cache
+    assert _texts_with_cache(doc) == ["그대로인 글꼴의 문단"]
+    reopened = HwpxDocument.open(io.BytesIO(doc.to_bytes()))
+    assert _texts_with_cache(reopened) == ["그대로인 글꼴의 문단"]
+
+
+def test_replace_font_in_other_blocks_only_keeps_unchanged_paragraph_caches() -> None:
+    doc = _decor_doc()
+    _cached_paragraph(doc, "가", "3")
+    _cached_paragraph(doc, "나", "0")
+
+    doc.styles.replace_font(DECOR, DST, langs=["USER"])
+
+    # charPr 3 names no decorative face; charPr 0's user face moved to DST
+    assert _texts_with_cache(doc) == ["가"]