In regards to changing terms on closed-end credit, would changes be permissible only with a formal Change in Terms Agreement?
You have to abide by the terms of the contract. In most cases these contracts (closed-end) cannot be unilaterally changed, therefore your borrower would have to agree to the change by signing a change in terms agreement. And if they refuse you have to honor the existing contract. I for one would not agree to such a change in my contract.
Also, one thing I don't see mentioned. The late charge will leave the payment short. What will be your policy for advancing the payment date. I pay my Nov payment 12 days late, you hold out a $17 late charge. Do you advance my payment to Dec since it is now $17 short? And what do you do if I make my payment on time in Dec for the payment amount since the previous payment was $17.00 short? Also, as Okie Banker suggests, this will cause the account to accrue additional finance charges on the unpaid $17. I would argue in court, and probably win, that you are charging interest on the late charge and you therefore are compounding the late charge.
You are opening a whole can of worms on the closed-end credits. You better insure that you have the full blessing of the bank's attorney.