You can work with Withers at your camp to completely rebuild any companion's abilities and class (this keeps things fresh while letting you experiment with different party compositions). The process costs 100 gold, and while you can change nearly everything about their combat capabilities, their core identity stays intact – that means their race and background remain the same because these are tied to their personal stories.

What's particularly interesting is how the game handles story consistency when you make these changes. Even if you transform Shadowheart into a mighty barbarian, she'll still maintain all her Shar-related dialogue and quest elements, since that's fundamental to who she is. The system actually adds some neat touches, like companions getting special voice lines when you make them Bards (something worth considering if you enjoy the roleplaying aspects).

A helpful detail to remember is that you're essentially rebuilding them from level 1, which means you can carefully plan their progression path and create exactly the character build you're envisioning. While the 100 gold cost might seem steep early on, it's actually quite reasonable considering the flexibility it offers (and you'll have plenty of gold later in the game).