* There is no equation between being a signatory and being a beneficial owner. You must ask the person opening the account to identify the beneficial owners; i.e. those individuals who own directly or indirectly 25% or more of the legal entity customer plus one individual with management or control responsibility. Whether any or all of them sign on the account is irrelevant.
* It's not triggered by a "new customer." It's triggered by a "new account." If a corporation that has banked with you for 25 years opens a new account after the effective date, the regulation applies.
* I don't know what "change" you might be referring to, but you can certainly identify events in addition to the opening of a new account where your bank would choose to require a new Appendix A. I don't recommend it.