i can tell u how to do it, do u want to know? u just need to export the model(use warcraft3 viewer, u can find it on this site), convert the exported file(should be *.mdx) to *.mdl. open it using wordpad and look for the path of the skin, change it to the skin path that u wnat, save it and convert it back to mdx.
remember to use a different path in the game, so it will not replace the original
this is how u change the skins of a model, u can do it everytime u dont want to replace a original skin