The Crescent tends to be one of Amtrak's more popular trains. So if you're traveling on a weekend, during the summer or a holiday period, then the odds of an on-board upgrade aren't good. If you are traveling outside of the above critirion, then the odds will go up.

Now as GoAmtrak said, if there is a sleeper available, then you would pay the price that Amtrak charged when that train was put on sale 364 days ago and that is always the cheapest price. You can't book more than a year in advance.