Originally Posted by
Lawwl
Quality or level has no impact to the transmogs. You can transmog shadowmourne into a level 1 grey weapon, if it's 2handed axe/sword/mace.
You need a special transmog scroll only obtainable with coins if you want your weapon to look like legendary. For example, if rogues want to transmog their weapons into illidan's warglaives, they have to purchase special scrolls in the store. But if you are transmogging a legendary weapon (such as shadowmourne or valanyr) into non-legendary one, a regular scroll works just fine.