Ability that changes damage type?

Is it possible to make an ability (like footman's defend, dwarven avatar, etc) that causes the unit's damage type to change for the duration? If so, how do I do it?

You could use a transform spell that transforms that unit into a whole different unit with the exact same stats except a different attack type...

ya, but i forgot to mention this is a hero...my bad.

Even easier.. If your using a hero you can also add a duration.. simply use metamorphisis, and heavily modify it of course..

Oh. Duh. Modify Metamorphisis. Wonder why I didnt think of that. 0_o

You can change a hero's armor or unit armor type by an upgrade effect.
Thats easier.

Upgrades cannot be down-graded. In other words, doing such wouldn't be reversible.

Definitely go with Metamorph -- you can probably just use a copy of the hero, but change the damage type. That's basically how it works anyhow, right? Examine the relationship between the Demon Hunter's morphed and unmorphed stats.