MischaZ: The current version of the nexus mod allows to set companions back to level 0 and set them to 25 points.
Technically you might even assign a new class/portrait etc. However I wouldn't touch this, that's begging for bugs and inconsistencies.
If unity mod manager doesn't work or you don't like to use stuff without being able to take a look at the code and what it does to your save, there is also a savegame-editor on github that allows to reset to level 1, change points etc. You can assign your companions to new classes via templates, but so far only base clases are available. You have to compile it yourself, but that's not difficult. Should work for Mac as well.
Just be aware that changing all your companions to the better (sub)classes really ruins the balance. For me, changing to hard difficulty was a decent solution to that, but if you are already playing on higher difficulty levels with the standard companions you might get bored quickly.
edit: Sorry, I'm not allowed to post links. The nexus stuff is easy to find and the save game editor on Github is made by ericfitzgerald. that's nice info. didn't follow that mod development. thanks.