I reached out to Reg DD expert, Rebekah Leonard, and the following is her response:
This is not a Reg DD issue, as Reg DD does not speak to error resolution at all. (In fact, it doesn’t even require you to send a statement! It only prescribes certain account disclosures if you do send one, and error resolution is not part of it. That falls under Reg E, which only covers electronic transactions.)
Check matters fall under UCC rules (which are governed by state law) or contract law. UCC 4-406 actually grants a customer one year after the statement date to discover and report an unauthorized signature or alteration. https://www.law.cornell.edu/ucc/4/4-406 Whether or not your state has adopted this provision of the UCC without revision is a legal matter for bank counsel to guide you upon. The same is true for guidance on the necessary notice to give for altering a contractual agreement (your Terms and Conditions) in your state. In short, these are legal matters, not regulatory compliance matters. Contact bank counsel for assistance. Good luck!