This worked great for me, on an Acela trip that I had booked using free trip coupons. I bid and was upgraded 2 hours beforehand on one trip and 1:58 beforehand on another trip. I just showed my ticket on the Amtrak app when boarding; it had been updated.

First class was 1/3 full even during a pandemic on a weekend so I wouldn’t count on inexpensive upgrades for long.