An email just went out about how this migration needs to happen, but it's not entirely clear how this affects App Engine Standard. I have a couple of small things running on that, and the only thing I see in the Container Registry is the app-engine-tmp folder.
Do we just delete that folder? Are new deployments from now on going to use Artifact Registry? More information would be great from anyone who might know!
Any project can be migrated from GCR->AR by running the automated migration tooling. This will keep the existing URLs and allow anything else that depends on GCR to continue functioning: https://cloud.google.com/artifact-registry/docs/transition/auto-migrate-gcr-ar
Previous answer (now out of date):The announced change does not affect any projects that are currently using Container Registry. Only projects not using Container Registry by May 2024 will be affected and they will use automatically use Artifact Registry for App Engine.
If you want to make a similar change to existing projects, you can follow the steps in https://cloud.google.com/artifact-registry/docs/transition/setup-gcr-repo Doing this will completely remove Container Registry usage from the project and use Artifact Registry for both gcr.io and pkg.dev urls (and App Engine)
-James (Artifact Registry Engineer)