Hi Jarnosrt,
1: Yes, but open a copy of the project. You can upgrade from 4.27 to 5.1, but cannot downgrade.
2: Lightmaps should be auto-generated, if everything is good in 4.27 I’d expect it to be good in 5.1 as well. (If you’re using Moveable/Dynamic lights you don’t need to worry about lightmaps)
Bonus: This community tutorial is the best resource that I know of for testing Lumen. (I’d recommend a 30 series GPU minimum)
Good luck!