Originally Posted by
Snowraven
Well, the forgetting part is easy. Warrior to warlock? Ok. The Warrior had its arm, leg and some ribs broken by a creature. It could not move for days so the bones started fusing badly. When he/she was found and healed, due to earlier bone and muscle fusing oddly, the warrior could no longer hold all kinds of weapons and perform his amazing feats without severe pain. He/she decided that they'd need to still help Azeroth somehow, and they will do this by learning of demons and using said demons as their protectors and damage dealers while they use their former hatred to churn spells they'll learn. In a proper setting, this would be something going on over months or even years, which is why it takes 1 year to change a class in my suggestion.
We can do the opposite too. Warlock had his pets abandon him/her to the Legion. He/she was found by a fellhound who sucked the magic out of them. He/she killed the fellhound with a random plank nearby and decided they would never be so weak anymore. They would learn to fight, as to not depend on creatures that can abandon them to the Legion. Even if the fellhound's effects remain and they can't cast spells anymore, they would still fight for Azeroth in a new way.
Also, you want a clear example. Since priests, warriors and paladins are too close so going from one to another might be considered cheating, how about the whole tauren sun druids? Druid to paladins and priests. And they forgat most things that made a druid... druid. Like, their paladins and priests can't become animals anymore or use the powers of nature.
Another example? Ner'zhul became from a shaman a warlock. He abandoned the shamanistic ways and became a full warlock. And he wasn't the only shaman to do this. Some came back to shamanism eventually, but many did not.
Delas Moonfang became from a priest a paladin.
Draenei learned how to become shamans from paladins. Nobundo was the first, it was easy for him being broken, but then he taught others too, non-broken draenei.
Death knights... all of them are former (other class here), they forgat everything and became death knights.
Demon hunters? They abandoned everything to become demon hunters.
In fact, these former 2 classes are the only ones that would be odd, weird or impossible to become other classes. Since death knights are undead and demon hunters are half demons so they couldn't become... quite a few other classes. They could probably still be switched among eachother, since a demon hunter can still die and be brought back as a death knight. and a death knight can use a demon soul to fill the void inside thus becoming a demon hunter.