There is no account type for user who only pays for the subscription. The change of rights is needed, and it is not quite easy to do so.
Available workaround (if any):
Change the Admin account. Difficult if there is only one PM/Admin for the subscription.
Add an Admin/Owner-only type of user, who would have only administrative rights/responsibilities and would not therefore count as PM account.
Some clients have a different person who purchases the edition. In case there is only one PM purchased, it causes difficulties with exchanging the rights simply to pay for the subscription.