If you're porting 2.1 wads, I have some good news for you.
In terms of maps, most of the thing types, linedef types, sector effects, etc. are the same from 2.1, except there are a few things that tend to not work. The biggest example being:
Zoom tubes
Speed pads
Polyobjects
Maces
Also, you'll have to convert most of the graphics from the 2.1 palette to 2.2 tho.
Lua and SOC stuff tho, you'll need to actually learn a bit of and whatnot if you want to port stuff there without it breaking.
And as for v2.0 and Final Demo, you'll have to do quite a bit of work to get these to work, ESPECIALLY in the case of Final Demo.