Hi Marius,
Please implement ability to reset a rebalanced/heavily nerfed Hero to return resources spent on this hero (skill books, chickens, potions) and reset hero to its base level. This may be time limited option. This way if someone is very unhappy with what you have done to hero, can undo his investment.
The whole problem is not about nerfed hero being viable after nerf. Its about a fact that we spent time/resources on hero we think will help us best at the moment. Even if hero stays viable it may not be best option out of the heroes people have to level up. Knowing what will happen to changed/nerfed heroes earlier they could choose different one to work with.