diff --git a/build-common.sh b/build-common.sh
index c1b0d98..a29f39a 100644
--- a/build-common.sh
+++ b/build-common.sh
@@ -109,6 +109,13 @@ cleanup_site_packages() {
     # Strip debug symbols from native extensions
     find "$sp_dir" -name "*.$ext_suffix" -exec strip --strip-debug {} \; 2>/dev/null || true
 
-    # Remove .py source files (keep .pyc only) — saves ~30-40% on pure-Python packages
-    find "$sp_dir" -name "*.py" ! -name "__init__.py" -delete 2>/dev/null || true
+    # NOTE: do NOT strip .py source files. A previous version of this function
+    # ran `find ... -name "*.py" ! -name "__init__.py" -delete` with a comment
+    # claiming "keep .pyc only" — but no compileall step exists, so the dist
+    # shipped with __init__.py + .pyd only, missing every submodule (Image.py,
+    # ImageDraw.py, _version.py, ...). Fresh installs would fail with
+    # ModuleNotFoundError; in-place upgrades over an older install produced a
+    # half-old/half-new site-packages where PIL/__init__.py was new but
+    # PIL/_version.py was stale, yielding the runtime "_imaging extension was
+    # built for another version of Pillow" import error.
 }
diff --git a/installer.nsi b/installer.nsi
index e7a9229..dc4ee10 100644
--- a/installer.nsi
+++ b/installer.nsi
@@ -70,6 +70,19 @@ Section "!Core (required)" SecCore
 
     SetOutPath "$INSTDIR"
 
+    ; Wipe previous payload before extracting so stale files from an older
+    ; version cannot survive an upgrade. Without this, in-place upgrades
+    ; produce a half-old/half-new site-packages — e.g. an old PIL/_version.py
+    ; alongside a new PIL/_imaging.pyd, which raises "_imaging extension was
+    ; built for another version of Pillow" at runtime. config.yaml lives at
+    ; $INSTDIR root and is preserved.
+    RMDir /r "$INSTDIR\python"
+    RMDir /r "$INSTDIR\app"
+    RMDir /r "$INSTDIR\scripts"
+    Delete "$INSTDIR\${EXENAME}"
+    Delete "$INSTDIR\VERSION"
+    Delete "$INSTDIR\config.example.yaml"
+
     ; Copy entire distribution
     File /r "dist\media-server\*.*"