Transmogrifying a weapon takes only the skin of the item. Stats on the base item reman the same. Speed doesn't count, but a dagger can be changed only to a dagger, sowrd to sword and so on. Also leather to leather only (WTF I WANTED SPAULDERS AND HEL FROM PRIEST TIER THEY'RE AWESOME)
EDIT: a challenge for everyone - rogue white-ish set.
Waiting for screenshots.