This depends on whether you want credits to be charged afterwards when you add a client to a series.
If you can charge credits afterwards, the client will be continuously scheduled in the series, even if the client has no more credits. You can then give a new product yourself and charge the credits afterwards.
However, this works differently when credits are always charged immediately and a client has a multi-ride card or an ending subscription. When you add the client to a series that is longer than the number of credits or the end of the contract date, the client will not be scheduled for the other sessions of the series.
If the client has credits or a new subscription again, you can easily add the client to the series again.