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.