Well, it is not impossible if you use Spellbooks, but that would be very hard.
An alternate option is to use the Tinker's Upgrade ability and make it an item ability. Set the damage/speed bonus fields to 0 (if you want) and then you can replace up to four of the Hero's abilities. If you want to replace less than four, remember to set the other fields to replace any X ability with X ability otherwise it causes problems.