I sortof feel like its not so much them changing jobs/classes, but its just that their natural combat styles evolving.

Thancred is a good example. Starting out using a dagger like a sword. Gets the job done, but not nearly as efficient as using either two daggers, or one sword. Then later on he starts using two daggers and he starts fighting more like a rogue.. And eventually, after all the fighting he's done throughout the game, he's now this badass ninja assassin.

I mean it makes sense. The more you practice and the more you fight, the better your combat style will get. He's just coming into his own, as they all are