What if I need coordinates in double precision? Large objects, Large planets, etc
Maybe you could work around it by faking that scale and actually scale your objects down. Depends on what do you mean by large objects, large planets 
To answer your question, you would need to rewrite quite a bit of engine code. Just change FVector from float to double and then fix all the potential errors that appear and you should be done.
That being said, you can make a space game with floats. You just need to fake a lot of things - you also need to fake a lot of things with doubles, they don’t magically make a space game of that magnitude happen.
One thing you should look into is world origin rebasing, that sets the player to (0,0,0) and shifts the world around the user so everything around the user can be simulated correctly without any floating precision issues.
I know this is really old, but isn’t there an issue with world origin rebasing where in the editor actors will just disappear/go to random places?