I basically agree with everything @zanos said above.
Additionally, allowing changes to companions' starting classes is yet another symptom of Larian's design philosophy, which I have serious problems with. If there weren't any other changes to 5e rules, then I (and I imagine other people) might be more accepting of this change. However, the combination of everything we know about BG3's EA design, plus the unknowns that we'll only find out on release date...it's worrying.
It's the design philosophy and game themes of: "choices about character options don't really matter because anything can be changed at any time; just have a wacky time casting multiple spells per turn, throwing bombs/halflings, and making use of other exploits we've added into the game." All together it indicates a non-committal gameplay style for BG3 that I don't particularly like.
For example, I can easily see this design philosophy extended to the end game: "even though you've made heavy use of the tadpole's powers throughout the game, succumbing to temptation, during the final battle/dialogue you can reject all that with no consequence or struggle" while tadpole non-users will have the same exact option, just without all the extra benefits of tadpole powers throughout the game.
