Trigger spell help!!!

-Spell name:Advance transform
-Description:Transform caster hero type random[2-4] after 60 second transform caster back to his right form type[1].
-I can replace em but when unit replace they must relearn their skill->it mean this spell has no cooldown :(
-How can i make this spell cooldown when caster has replace back->i trying reset ability cooldown but it wont work->please help me or send me a tutorial map about this ^_^

First thing that comes to mind is use the metamorph ability, but make a few copies of the same ability.

Periodicly with triggers, remove 1 version of the ability and replace it with a different, each different version will morph the hero into a different unit.

Swaping the abilities will be unnoticable to the human player, just make sure you do not swap while it is cooling down, other wise you will clear the cooldown.