This is more complicated than it should be, and it is on our list of future improvements. To put it simply, you can't change the admin if the account is linked to an active subscription.
The subscription must be canceled, and the current service period must expire before you can switch the admin.
Additionally, the new admin account cannot be listed as a "guest." You’ll need to remove the Gmail account first, then use the "replace admin" feature once the account has no active subscription.
In special cases, such as when a user no longer has access to their email for password recovery, we can assist with replacing the admin account on our end. However, this requires the subscription to be canceled first. After the admin account is replaced, the user will need to re-subscribe under the new admin account if they wish to continue using the add-on services. Please note, we are unable to provide a prorated refund for any unused services at this time.
