Question: Regarding Reg CC and check holds, can we apply a new account exception hold on an established account when a new account holder is added?

Answer: Interestingly enough, yes.  Here is the commentary from 12 CFR 229.13(a)

(1)(b)(vi): If two customers that each have an established individual account with the bank open a joint account, the joint account is not subject to the new account exception. If one of the customers on the account has no current or recent established account relationship with the bank, however, the joint account is subject to the new account exception, even if the other individual on the account has an established account relationship with the bank.