Hi,
It is several years since I used a rail pass myself, so my info may be out of date...
You can't reserve any tickets with a rail pass on the Amtrak website, but you can buy the pass online.
As to the cost of upgrades, the pass will cover the "coach seat" element of the journey, so you just need to pay the extra.
At one time that was quite easy to work out from the Amtrak website, but nowadays there are several prices for coach seats shown.
I assume that the basic "value" fare is the regular price, so deduct that from the price shown for a roomette, and that would be your cost to upgrade.
I have used a random date of August 20th, 2019 as an example for your trip from SLC to Denver. The value fare for two adults is currently showing as $168. A roomette is priced at $350 for two adults. If you take away the price of the value coach seats, which your rail pass would cover it would leave $182 to pay for that upgrade.
There are several things to be aware of with the rail pass...
You buy the pass online, they send a confirmation number to your email address.
You then need to contact them in America and state which trains you want the tickets for, and any upgrades you require.
There is no guarantee that your desired trains will still be available, as the pass only covers you for the lowest "bucket" price, Amtrak prices of coach seats and sleepers rises according to how many are sold
You pick up the rail pass and all the tickets once you arrive in America, from an Amtrak ticket office at the station.
I used to use the pass frequently on my own trips to America, riding only in coach, but the last few times I just bought point to point tickets on the Amtrak website, the rail pass seemed hard work in this digital age!
Ed.
PS, Please check that I have got things right, things may have altered since I last used one!