Make sure you read the release notes thoroughly, particularly the first part on upgrading from earlier versions
This is a pretty substantial change, so I'll just include some highlights here:
- All the faction soldiers can now get Master Sergeant and have extra perks
- Chosen are now enabled through a Second Wave option rather than a INI setting
- The chance for Chosen to appear on a mission is affected by the infiltration level (the more you underinfiltrate, the more chance for them to appear)
- The LW2 flamethrower targeting is back! No more frustration as you fail to hit any targets with it because you're standing behind some half cover.
- The invasion mission is a little easier on the reinforcements, but you can no longer one-shot the spike to stop them
- The Squad Select screen has some quality of life changes, including the map type the mission will be on (so you don't get stuck bringing a Death-from-Above Sharpshooter on Tunnels).