If you are having trouble saving a payment plan change, there are a couple things you should look out for:

1: The Distribution Type should be selected, per the fee/discount setting, for all highlighted bill lines.
distribution type

2. Do not change any billing amounts. You will have the opportunity to make updates in Edit Billing Details after the plan change process. The Change Plan button will not function if you made any billing edits.
billing amounts

3: Enter only letters and spaces for the reason. Characters or numbers will give you an error.
reason error