I understand that it could be tricky assigning Companions to new classes if their personal story is tied to their class, such as Shadowheart or Warlock. So I'm not here to talk about that.
I hope that it will eventually be possible to re-assign the existing ability scores for companions - not re-roll new ones, just move the old ones around. Right now, for instance, Astarion makes a poor Arcane Trickster because of poor int. Same with Lal'azel and Eldritch Knight - her INT is bad, so she's not going to make the best EK.