What's the process for true vendor name change request?

I don't change vendor names until I have an invoice because if it is just the corporate name that is changing or the business is acquired, you don't know what name they will be using for billing.

Always request a new W9 if a vendor requests a name change.  If it is the same EIN number, then it is truly a name change and the vendor's file can be changed.  If it is a different EIN, then it is not the same vendor and should not be changed.  A new vendor number with the new name and EIN should be created.