When the game comes out? for example if you wanted to change Astarion's hair colour or Laezal's face?

No one knows but I really don't think so. Some of them already appear in cinematics (Lae'zel in the openning)

True, but they are not allways similar to themselves ...
It honestly pisses me off that Lae'zel wears slightly different armor in cinematic and in game.
(Focus on shoulder parts)

I would expect that you will be able to. They allowed this in DOS II and Larian likes giving players choices. Don't see cinematics making a difference as characters in cinematics wear the armour you equip them with.

Probably. In DOS2 you could alter them as much as you wanted within their locked race, and I'd expect the same to be true for BG3.