Replacing a 2017 logic board with a 2019 or 2020. Is it possible?

Unless you can do this for substantially less than buying the whole computer, I would probably replace and sell the machine while you have casual buyers who still want to buy Intel hardware. The problem isn't that it can't be done providing compatibility of the display is a non-issue; the issue is that the remaining Intel Macs Apple supports are nearing their EOL in ~1-2 years (if not sooner). Once that happens they nosedive in value every time outside of the 2019 MBP, 2018 Mini, and the 2019/2020 iMac for some reason, at least right now.
The main difference between 2019 and 2020 is the 2019 version will likely stop at Sonoma officially but lacks a T2 SoC. The 2020 iMac may get one more MacOS upgrade, but that's almost certainly the end; Apple is dropping Intel like they did PPC in 2006, but at a better pace this time around. Take advantage of the false economics and get out of it while you're ahead. I wouldn't pay more then near scrap if it was Intel, but there are plenty who will pay absurd eBay prices for these things.

As long as the display is compatible with the 2019/2020 logic board, you're good to go -- but keep in mind the 2020 iMac logic board has soldered RAM on the 21.5" AND the SSD is soldered by the nature of it having a T2 SoC.

Your money is better spent on an AS Mac with 16-32GB and 1-2TB of SSD storage.