I think if you changed your index from "prime minus 2" to "prime" your customers would be quite pleased.
However, I don't think that's what you meant to say; your desire is to change the index to the customer's detriment; e.g. "prime minus 3."
There have been a number of recent discussions about raising early withdrawal penalties during the life of a time deposit. My observation has been to the effect that it's an unfair and deceptive practice on its face. My reaction here is the same, but would be a bit stronger.
Regulation DD might not interfere with your desires; i.e. you might be able to comply with the regulation in disclosing the change. Your concern would be with state laws that would take a dim view of your unilaterally changing the terms of the contract in an effort to deny the customer the benefit of the bargain. The label used by your attorney general's office would probably be "bait and switch."
P.S. If you work for my Mom's bank, plan for a class action suit.