You can find the process of project migration here, please be advised that this procedure only involves migrating projects from one organization to another. If you want to transfer Google Workspace domains between organizations, it’s recommended to reach out to your Support representative, please take in consideration if you are planning to migrate your project to a different organization it’s recommended to follow the project migration checklist that cover this points:
1.- Create a migration plan
2.- Address any special cases
3.- Assign Identity and Access Management roles
4.- Set organization policies
5.- Perform the migration
In order to change the billing account for a project you can follow this procedure, regarding your API key concern I trust that Analyze Move API may help you to get a detailed report on warnings and blockers from a list of critical policies systems prior to actually moving your project