Add instructions for already on vite for upgrading past 6.7 (#1285)

Co-authored-by: kategengler <444218+kategengler@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: kategengler

docs/upgrading-past-6-7-0.md

@@ -44,6 +44,42 @@ Re-run `ember-cli-update` after that.
 
 ## Moving to Vite
 
-Use [ember-vite-codemod][mod] to upgrade your project.
+Use [ember-vite-codemod][mod] to upgrade your project. Then see [Already on Vite].
+
+## Already on Vite
+
+[Already on Vite]: #already-on-vite
+
+You will also need to update
+`config/ember-cli-update.json` and replace the base blueprint:
+
+```diff
+@@ -2,13 +2,11 @@
+{
+  "schemaVersion": "1.0.0",
+  "packages": [
+    {
+-      "name": "ember-cli",
+-      "version": "6.7.0",
++      "name": "@ember/app-blueprint",
++      "version": "6.7.1",
+        "blueprints": [
+          {
+-          "name": "app",
+-          "outputRepo": "https://github.com/ember-cli/ember-new-output",
+-          "codemodsSource": "ember-app-codemods-manifest@1",
++          "name": "@ember/app-blueprint",
+          "isBaseBlueprint": true,
+          "options": [
+            "--ci-provider=github"
+          ]
+        }
+      ]
+    }
+  ]
+}
+```
+
+Re-run `ember-cli-update` after that.
 
 [mod]: https://github.com/mainmatter/ember-vite-codemod/