I am trying to locate the guidance on when (timing of) and if a notice has to be provided to the borrower on a closed-end ARM loan when switching out from LIBOR. Can anyone point me in the right direction, Reg Z??

If you have loans on which you need to make the transition to a new index, the regulators have been warning banks of this approaching for some time now. Many bank have an entire task force established to handle the transition for all area within the bank this will impact. Here is one, but there is a lot of reference materials from the regulators out there.

To your actual question though. of course you have to notify your borrowers. .

https://www.ffiec.gov/press/PDF/FFIEC%20Statement%20on%20Managing%20the%20LIBOR%20Transition.pdf

"Institutions should understand the legal, operational, and other risks they face associated with various consumer financial products as a result of the LIBOR transition. Institutions should plan appropriate actions to address or mitigate these risks. Transition plans should identify affected consumer loan contracts, highlight necessary risk mitigation efforts, and address development of clear and timely consumer disclosures regarding changes in terms. Disclosure of these altered terms should, and in some cases are required by law to, be communicated to borrowers in advance of a reference rate change to help them understand how a new reference rate affects their contractual principal and interest payments, APR, and other terms.