If I were in your position, I'd book the sleeper now. It's at a decent bucket. Bedrooms are soldout, but roomettes are at $371 a piece. Keep in mind some conductors are just too lazy to do the paperwork!
Honestly: calling Conductors "lazy" is a little insulting. I would say selling an onboard sleeper upgrade to someone who is too cheap to buy one in advance is LOW on the priority list, somewhere behind Train Safety and Eating Popcorn.
Hey, if the onboard upgrade thing is your gig, sorry to insult ... but I would never expect a Conductor to drop everything to serve that request.....