What changes if we allow an existing investment property mortgage transfer the deed from the individual name into an LLC?

Changes to what? Sounds like a legal question.

Our customer is purchasing several rental properties and wants to change the name on the Deed of this existing loan from his personal name to his newly created LLC name...

If nothing else changes except the name on the Deed does that make our note invalid at that point?

I'm thinking no, since the obligation still falls on the individual, the property is just being held in the LLCs name... I hope that makes better sense...

You are asking a legal question and much of it will be based on State law You need to ask your legal counsel for guidance on the best way to handle this.

We did something similar once. We needed a new mortgage, and since our documents referenced mortgage information, we re-documented the whole loan. We involved a title company and got an MRT endorsement (because it was deeded and it didn't make sense to pay MRT since no new funds were advanced and new mortgages are generally associated with new money). There could be another way, that was just how we handled it in our state.

I agree with rclarey of course, legal should probably be involved.