diff --git a/.github/workflows/build.yaml b/.github/workflows/build.yaml
index 4992232..2d83545 100644
--- a/.github/workflows/build.yaml
+++ b/.github/workflows/build.yaml
@@ -234,6 +234,12 @@ jobs:
         if: matrix.os == 'windows-latest'
         shell: bash
         run: |
+          # MSI ProductVersion only supports three numeric fields (major.minor.build, each
+          # 0-65535) and cannot represent a SemVer prerelease suffix (e.g. "-beta.12"), so
+          # it's simply truncated off here. Package.wxs's MajorUpgrade AllowDowngrades="yes"
+          # is what makes this safe: it always replaces whatever version is installed -
+          # older, same, or newer - so two builds truncating to the same MSI version (e.g.
+          # a prerelease and its eventual stable release) is not a problem.
           MSI_VERSION=$(echo "${{ inputs.version }}" | cut -d'-' -f1)
           echo "MSI_VERSION=${MSI_VERSION}" >> "$GITHUB_ENV"
           echo "Computed MSI version: ${MSI_VERSION}"
diff --git a/src/DemaConsulting.SysML2Workbench.Installer/DemaConsulting.SysML2Workbench.Installer.wixproj b/src/DemaConsulting.SysML2Workbench.Installer/DemaConsulting.SysML2Workbench.Installer.wixproj
index ed75346..e35a76d 100644
--- a/src/DemaConsulting.SysML2Workbench.Installer/DemaConsulting.SysML2Workbench.Installer.wixproj
+++ b/src/DemaConsulting.SysML2Workbench.Installer/DemaConsulting.SysML2Workbench.Installer.wixproj
@@ -23,6 +23,18 @@
     <!-- Exposes MSBuild properties as WiX preprocessor variables ($(Version), $(SysML2WorkbenchPublishDir),
          $(SysML2WorkbenchIconFile) in .wxs). -->
     <DefineConstants>Version=$(Version);SysML2WorkbenchPublishDir=$(SysML2WorkbenchPublishDir);SysML2WorkbenchIconFile=$(SysML2WorkbenchIconFile)</DefineConstants>
+    <!-- ICE61 flags this product's Upgrade table as having no Maximum version - that is exactly
+         what Package.wxs's MajorUpgrade AllowDowngrades="yes" intentionally configures, so any
+         version (older, same, or newer) always replaces what's installed instead of ever being
+         refused (see https://docs.firegiant.com/wix/schema/wxs/majorupgrade/). This ICE is a
+         known false positive for that setting and must be suppressed rather than acted on. -->
+    <SuppressIces>ICE61</SuppressIces>
   </PropertyGroup>
 
+  <ItemGroup>
+    <!-- Provides the WixCloseApplication custom action used to prompt the user to close
+         a running SysML2 Workbench before the installer replaces its files. -->
+    <PackageReference Include="WixToolset.Util.wixext" Version="5.0.2" />
+  </ItemGroup>
+
 </Project>
diff --git a/src/DemaConsulting.SysML2Workbench.Installer/Package.wxs b/src/DemaConsulting.SysML2Workbench.Installer/Package.wxs
index f29a34a..4312be8 100644
--- a/src/DemaConsulting.SysML2Workbench.Installer/Package.wxs
+++ b/src/DemaConsulting.SysML2Workbench.Installer/Package.wxs
@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs"
+     xmlns:util="http://wixtoolset.org/schemas/v4/wxs/util">
   <Package
     Name="SysML2 Workbench"
     Manufacturer="DEMA Consulting"
@@ -9,13 +10,42 @@
     Compressed="yes"
     Scope="perMachine">
 
-    <MajorUpgrade DowngradeErrorMessage="A newer version of SysML2 Workbench is already installed." />
+    <!-- AllowDowngrades="yes" means installing SysML2 Workbench always replaces whatever
+         version is already present - older, same, or newer - rather than ever refusing
+         to install or leaving two side-by-side entries in Add/Remove Programs. This also
+         covers the same-version case (WiX docs: "AllowDowngrades already allows two
+         products with the same version number to upgrade each other"), so it must not be
+         combined with AllowSameVersionUpgrades. -->
+    <MajorUpgrade AllowDowngrades="yes" />
     <MediaTemplate EmbedCab="yes" />
 
+    <!-- Closes a running SysML2 Workbench before install/upgrade so file replacement never
+         fails because the app is open. Sends a close message first (giving the user a chance
+         to save work), then forcibly terminates it after Timeout seconds if it hasn't exited -
+         RebootPrompt must be "no" to combine with TerminateProcess. -->
+    <util:CloseApplication
+      Id="CloseSysML2Workbench"
+      Target="DemaConsulting.SysML2Workbench.Desktop.exe"
+      Description="SysML2 Workbench is running and will be closed to continue installing."
+      CloseMessage="yes"
+      RebootPrompt="no"
+      TerminateProcess="1"
+      Timeout="5" />
+
     <!-- Sets the icon shown in Add/Remove Programs (Apps & Features). -->
     <Icon Id="AppIcon" SourceFile="$(SysML2WorkbenchIconFile)" />
     <Property Id="ARPPRODUCTICON" Value="AppIcon" />
 
+    <!-- Points Add/Remove Programs' support/about links at the project's GitHub page. -->
+    <Property Id="ARPHELPLINK" Value="https://github.com/demaconsulting/SysML2Workbench" />
+    <Property Id="ARPURLINFOABOUT" Value="https://github.com/demaconsulting/SysML2Workbench" />
+    <Property Id="ARPCONTACT" Value="DEMA Consulting" />
+
+    <!-- There is no WixUI dialog sequence, so the "Modify" and "Repair" buttons in Add/Remove
+         Programs have no UI to drive - hide them rather than offer non-functional options. -->
+    <Property Id="ARPNOMODIFY" Value="1" />
+    <Property Id="ARPNOREPAIR" Value="1" />
+
     <Feature Id="MainFeature" Title="SysML2 Workbench" Level="1">
       <ComponentGroupRef Id="ApplicationFiles" />
       <ComponentRef Id="ApplicationShortcut" />