The current keys/beta for the game will update to the full version.
There will likely be enough changes that keeping the beta installed would not help much with the full version download size. Even just reorganizing the current data could potentially change the compressed pak files by quite a bit, let alone tweaking various stuff, adding the other 2 origin characters and related quests and dialogue, etc.
You likely have a Kickstarter tier that includes 2 copies of D:OS 2 (such as the Kickstarter Collector's Edition tiers). That is a separate key you can request and give to someone to play co-op with, etc.
