Hexen lord thingi, you can't just do that, but maybe th15 is right, i have to change some variation things, i'll post back later to see if i made it work
Variations just tack on a number to the end of the original model.
For example, you have tree.mdl and 3 variations.
Variation 1 would be tree2.mdl, 2 would be tree3.mdl, and so on. Set variations to 1 and change your model.