Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,182
Do it at the hotel.
Don't ask to "change the account". Ask to change the "registered guest" and provide the new account number.
If the name of the registered guest isn't changed and you provide a Marriott number from somebody else, nobody will earn points / elite nights. Also note that officially the account holder must pay for the room - this may or may not be enforced by your hotel.