The issue here is the more frequent debit and not a limitation set under Reg E. I would believe the plan itself that is changing would have a responsibility to notify the consumer that payments are changing from annual to monthly.
If the plan or entity authorized to make the debit fails to do so, the consumer may then claim it as an unauthorized withdrawal. If it is the bank itself making the withdrawal, it should tell them, not as the bank and under Reg E, but as the ex-employer providing the health benefit plan.
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell