I believe you make your reservation for a sleeper when you buy your tickets. You 'probably' could upgrade at a later time as well, but I've never tried this AND if you're traveling in a busier season, you may lose out on a sleeper.
Furthermore, if you want to take the chance, you could purchase coach seats and upgrade to a sleeper at the station if there are sleeper's available. Amtrak will charge an upgrade fee. (No free upgrades here!). We usually purchase the standard sleeper and attempt to upgrade to the deluxe sleeper at the station.
As most people will tell you, the farther out you book the Auto Train, the cheaper the tickets usually are. You can book about 11 months in advance.
Hope this helps!