I’ve opened several projects in 4.2 that were made/downloaded with 4.1. It asks if you want to upgrade them in place, or open a copy of them. Mostly, they convert and work fine… the only issues I’ve had were ones that use code, it sometimes doesn’t compile with VS properly.
The only thing I’ve not gotten working as of yet is the full Unreal Tournament 4.1 in 4.2, but I gave up after about 10 minutes… it’s complaining about a DLL that the shooter complained about, but moving that DLL doesn’t help Tournament like it helped the shooter, so I stopped looking (it was 2am and I needed to get to bed 
).
I’m going to try and get back into it tonight, but “generally speaking”, upgrading them is fairly painless. Though, I suggest making a lower version copy until you get it loaded/working in the higher level system… once the conversion successfully completes, they say you can’t open the project again in the lower version (and if it requires a source code compile, once the compile breaks it’s no longer valid for any editor).