Originally Posted by AlanB

The cost for a roomette is good for 2 people. You don't pay per person for the roomette. Or to put it another way, one person pays for the roomette and their railfare (the amount Amtrak charges to go from point A to point B). If a second person joins the first, they only pay the railfare charge. There is no extra charge for them to occupy the room with the first person.

So if you upgrade on board the additional cost, assuming that the prices don't go up across the board before your trip, would be $200 more than what you've already laid out as a couple.

I've never tried to upgrade an AGR reward, so I'm not sure if that is permitted or not.