Good question, and unfortunately I think the answer is the basic reservation answer on this forum. It depends on your res. agent. You will have to call (this cannot be done online with a reservation for two people), call the AGR line if possible (register if not already), then emphasize that you want to “modify” your existing reservation by adding a roomette. At this point they should be able to just add at the existing rate and not affect your existing reservation. Then, if you don’t get the answer you were looking for, cancel the changes, wait a minute, call back, and talk to a different agent.
I did just this on the trip I am currently on for our upcoming SLC-EMY leg. My problem is I asked if I could add the room with points, which is not possible on the same reservation, and the agent got hung up on creating a new reservation so now we have two, but that’s still not a big deal. The good part is the roomette price had actually dropped, so I was able to get the refund amount from the drop and apply all that to the new room, so we got another roomette for minimal cost.
Good luck!!
