Although we don't do a lot of credit card payment plans, it is my experience that you are not allowed to edit an existing plan. It does make logical sense - the patient and the office make an agreement at the outset of the payment plan, forms are signed, etc. You should not go back and change the terms after the fact. What we have done as a workaround would be to terminate the existing plan and start anew.
Is there a way to add a credit card payment plan for multiple patients in the same family? It looks like it only adds it for the patient screen you are in.
Use the hot key "y" for payments - you should be able to access that screen from the transaction screen when making a payment. There is also a setting in Softdent that defaults to the payments screen when making a credit card payment. This allows you to distribute the payment to multiple patients, if needed, in the account.