The main change in this update is the introduction of multiplayer. Note that multiplayer on the original games is separate from multiplayer on Episodes – to play together, ensure that both you and your friends are all running the same ‘flavour’.
There is also much improved support for Steam Remote Play Together – namely, you can now hot-plug controllers while you’re playing and the game will use them on the fly (this capability is required for Remote Play Together to function correctly). You no longer need the workaround of changing the game’s language after your friend connects (only holds for Episodes – you still need the workaround for the old non-episodic versions of the games).
Also, Steam achievements now work.
You will lose:
any mid-mission progress
anything you’ve set in the Options menu
More information about this update can be found here: https://forum.chickeninvaders.com/t/episodes-v-35/39316