Anyone who can login using the main email address on the account has Primary Admin access. If you are able to login using the main email address on the account, then you can change the email address on the account to essentially change who has Primary Admin access.

After logging in with the main email address on the account, you can follow the steps below to change it, if needed.
Go to MembershipWorks > Organization Settings.
Change the email address in the Email field on the right.
Click Save & Continue at the bottom of the page.