No can do. An installment plan is a fixed promise and based on the originating order. If a subscription is adjusted: the frequency changed or the size is changed, that will not adjust the installment plan. In all situations, you need to calculate what is the total due v. what has been paid to date.
As the installment billing is ongoing, you have several options.
1. Cancel the installment plan and bill manually.
2. SIZE CHANGES: Continue billing via the original installment plan amounts and make adjustments to the member cash account: if subscription size is changed to smaller add the 'overpayment' to cash. If the subscription size is changed to larger add the amount 'extra' due and Bill for Cash from the member's cash account.
3. FREQUENCY CHANGES: If subscription changed from weekly to bi-weekly you owe the member. If subscription changed from bi-weekly to weekly, you are owed money.