As the initial custodian was opening the account for a person under a disability, he or she was the "customer" and CIP was mandatory. The CIP FAQ's
do not address your question directly, but Question 2 on page 4 indicates a co-owner being added to an existing deposit account is a "customer" and question 3 on the same page indicates that a new borrower who is substituted for an existing borrower is also a "customer" and CIP must be performed on both.
Ken, While I agree with your thought process and conclusion here, I don't think it applies to the successor custodian scenario.
In PA (and other states may be similar), an UTMA account can only have on custodian at a time. A successor custodian may be named, but the successor has zero authority to manage the account unless the original custodian dies or is incapacitated. So, the successor doesn't fit the regulatory definition of "customer". However, in the event that the custodian dies, and the successor takes over, the successor would become a customer, subject to CIP, at that point.
That said, you may certainly require that a successor custodian be CIPed at the time they are designated as the successor. And this is probably an efficient move, since it eliminates tracking of a separate trigger event later in the account cycle.