They have a pre-purchased EPIC version, we are eligible for a free STEAM or GOG version (optional). What is the procedure like, how will we be notified about it?
If you bought the game on the Epic store you are not eligible for the free steam key. Those are for people who backed the game before PP became the Epic exclusive, and were promised a Steam/GOG key. The offer only extends to purchases made before March 12th, 2019.
I expect those will be send out to eligible backers in similar way Epic keys were sent on or before Dec 3rd.
"Phoenix Point launches in just a few short days. As a backer, you are one of the people who made this project possible – we can’t thank you enough for your support. It truly means the world to us.
While Phoenix Point will not be available to download until December 3, 2019 at 11 AM Pacific Time, we want you to be able to register your key on the Epic Games Store and get ready for the game right now. Below, please find your key(s), along with the Backer Tier at which you supported us.
Your Purchase: *Signature Edition"
I really can’t remember from top of my head what Signature Edition was and when it was introduced and discontinued. Check when you bought the game and where. If you bought the game before Epic exclusivity was announced (march 12ve, 2019) you will get the extra key. You also should have been given season pass for free. This offer was meant to soften the blow of the game becoming an Epic Exclusive, while backers were let to believe earlier that they will receive steam/fog keys.
If you, however, bought the game after March 12ve 2019 when it was known that Phoenix Point is Epic exclusive release for a year, then you just got what you paid for - the game on Epic.