Originally Posted by
escapefromphl
Typically yes, as long as both people are present. You might be able to do this calling up Marriott as well if you can get a good agent.
The agents that can actually make this change do not take phone calls from guests, so you will be put on hold while the agent you're talking to calls the other agent. It is easier to do it at check-in. Either way is still a valid option though.