Hi Christian,
Can you please clarify the following:
1- Can you confirm it is an App Engine and not a Compute Engine app? This is because AppEngine application’s region is not modifiable after creating the instance.
2- Are you talking about migrating an instance of a single project or in-between multiple projects?
For GAE, we no longer support quota increases for the Mail API, as a workaround you can use a third-party mail provider, such as SendGrid, Mailjet, or Mailgun, which have higher quotas [1].