It's not possible to migrate existing extensions between miniExtensions accounts.
1. Change the account's owner
If you simply need to change the ownership of the account, you can just change the account's email address to something else and reset your password.
2. Bought a subscription for the wrong account
If you upgraded an account different than the one you intended, the solution is to buy another subscription using your other account (the one you plan to keep), then ask us to cancel and refund your first account.