I have been searching but with no luck. Does a regulation exist that requires the Bank to notify their Regulator (OCC) that they will be undergoing a system conversion to a new core processor?
Any help is appreciated.

Unless you are a de-novo or under an agreement of some kind I am doubtful you will find such a requirement. That said, that's something I'm sure your EIC (examiner-in-charge) would really like to know.

Actually, I believe the Bank Service Company Act requires notification regarding certain services that are outsourced. I can't tell from the OP if the new "core processor" is just the application, or actual servicing of accounts.

In any case, the BSCA is here.

And a FIL from the FDIC regarding this is here.

We have filed these forms for our internet banking provider, our ATM network provider, our online mortgage application provider and managed security services (firewall and IDS) provider.