indeed, is not possible to move the region of an app once is set, the documentation states that:
You cannot change an app's region after you set it. App Engine Locations.
But, as well states that:
Cloud Storage Location
When you create an app, App Engine creates a default bucket in Cloud Storage. Generally, the location of this bucket is the region matching the location of your App Engine app.
regarding buckets, seems that is possible to rename it and move it to a different region, so you can give it a try moving your bucket back to free tier and see if that help with your billing, otherwise, as stated in the previous response, you will have to recreate your app basically from scratch.
Moving and renaming buckets
When you create a bucket, you permanently define its name, its geographic location, and the project it is part of. However, you can effectively move or rename your bucket:
--If there is no data in your old bucket, delete the bucket and create another bucket with a new name, in a new location, or in a new project.
-- If you have data in your old bucket, create a new bucket with the desired name, location, and/or project, copy data from the old bucket to the new bucket, and delete the old bucket and its contents. The steps below describe this process.
If you want your new bucket to have the same name as your old bucket, you must temporarily move your data to a bucket with a different name. This lets you delete the original bucket so that you can reuse the bucket name.
Moving data between locations incurs network usage costs. In addition, moving data between buckets may incur retrieval and early deletion fees, if the data being moved are Nearline Storage, Coldline Storage, or Archive Storage objects.
Regards.
