If you want to transfer one paid subscription from one email account to another here's what you will need to do.
Example: John has a paid subscription with an email address of email@example.com and John leaves our company. I want to transfer the subscription and the email designs from John's account to Mary's account firstname.lastname@example.org.
Step 1: Cancel John's paid subscription (you'll receive a prorated refund for any unused part of the month)
Step 2: Create an account for Mary and take out a new subscription for Mary
Step 3: Move John's Sent emails to Mary's account so she start using them.