I'm not aware of any way to edit portrait assignments with an editor or via the console, or edit a saved game in any fashion, but you could change them the bastardized way -- which is to simply replace the appropriate .tga files in art\interface\PORTRAITS with whichever ones you'd like them to be. In other words, replace "studlyhumanfighter_big.tga" etc. with "sexpotelvinsorcerer_big.tga" or whatever, appropriately renamed to match the original. Be sure to back up.
it is posible to edit them I've done it myself - I did however only change the image and left the image filename the same.
