I believe it will be the case in the future. Swen talked about it in an interview, and they definitely intend to allow the change of classes for companions.
Absolutely not a fan of "changing classes for companions", by the way. Not in D&D, for sure.
I feel like their class and eventually their special abilities should be an integral part of their identity, not some accessory dressing that could be changed on a whim. I'd obviously prefer just having more companions to select from. Then again Larian is making this exceptionally hard for itself with this terrible idea of making "every companion also a possible Origin story", which inflates the cost of creating each one considerably. Something that I hope they'll reconsider.
I'd take having a large selection of interesting characters over "having just few of them and being able to replay the game in their role" any day, frankly.
I have to agree that changing any characters class be it companion or the player mid game would just feel wrong and is something I would never do. The only time I could ever see me using such a feature is as a respec option so I could alter some of their stats and abilities within the same class depending on how Larion built them or if I fudged them in levelling.
There will probably be twelve origin characters at launch, one for each planned class and that's probably enough, your right if they make to many origin characters it's too much unnecessary work, but then I would also expect more companions that can be recruited in later chapters to give multiple options to pick from for the different classes who will have different personalities, alignments and skillsets.