You are going to have a very difficult time with the transfer as it can only be done on the backside by Databricks.  Your only real option would be to have your customer create their own account and migrate the workspace assets over outside of having a support contract.  Hind sight being 20/20, it is always best to have the customer create their own accounts and cloud assets for a number of reasons including integration into their IDPs.  Once done, have them provision you a user account and then promote to Account Admin so you can conduct the work you need to do.