By adjusting the Credit Validity of an existing membership, it will also move the Period Date of the credit line within the membership. It should be noted that there isn't currently a way to bulk edit the validity of all or multiple Credit Lines at one time and therefore this can be quite a time consuming task. A quicker solution would be to cancel and reallocate a new membership to the client's account and ensuring that upon processing the new account the First Credit Allocation Date is set to the desired day. This will also ensure that all credit validities align with the schedule payment dates. 

Use the below step by step guide to amend credit validity.