I'd be pretty surprised if there were any bedrooms available on the first leg to DC, as the Viewliner sleeping cars only have 2 per car. So they tend to sell out very fast in the summer.

The odds would go up on the Capitol from DC to Chicago, or the SW Chief out of Chicago. Even still, it is peak travel season so I certainly wouldn't hold my breath either. However, if you are lucky enough to find an empty room, the upgrade price could be very nice. In fact, since they have to charge you the lowest bucket price when you upgrade onboard, it is actually possible that you have paid more for the roomette than the lowest bucket price for a bedroom.

In that case, it would cost you nothing to upgrade. Even if it's not lower, I would expect that the extra fare wouldn't be all that steep. So it's definately worth asking if there is an empty bedroom.