Change of Activation Date
Although you can move the session credit activation forward/backward by following the steps below, you would also need to manually create invoices for payments not generated prior to the original date you had it set up. Sample Image below.
Another option that I can suggest is to cancel the membership and just allocate a new one.
Steps to update Credit Activation Start Date:
Go to Client Profile Page> Payment/Credit card icon> Locate Active Membership/Package> Click on the View History >Click on Update Credits> Enable Edit Credit Validity toggle> Set the Start date as per your preference > Save & Activate.
Please see the below step-by-step: