You'll have to contact either Amtrak itself, or a travel agent, in order to determine if there is still accomodations for you to upgrade to. The other options are ( and this is a serious answer) to wait until 4:30 am of the day of your departure, call Amtrak, and see if you can get a hold of spaces that have just become available due to another passenger's cancellation of their trip. You can also wait until you show up on the train & make enquiries to the persons listed above. You'll be travelling on the Empire Builder, which is known for having being heavily booked throughout the year with no "off" season. I've known several individuals who were able to get accomodations at the very last moment because the Amtrak computers don't list the day's cancellations until approx. 4:30 am CDT. You'll also be responsilbe for paying the difference in price, obviously, between coach & sleeper class. If you really wanted a sleeper aboard the Builder, you would be advised to go ahead and simply get the sleeper tickets, instead of having to wait until the last minute & be forced to use the options I have just described.
On the other ahnd, there are very few things that get the adreniline pumping like trying to get accomodations and catch a train at the very last minute. Best wishes in your endevours. When I made reservations for my trip aboard the Builder this weekend, I had to get the tickets weeks in advance.