Lua, save to scripts folder:
Run to characters database:
To transmogrify an item you need to have the item equipped you want to change.
Then you need to have an item of the same type (inventory type to be exact) somewhere in your inventory (doesnt matter where, just in some bag).
When talking to the NPC you get to select the slot.
After selecting you see a menu with item names of the items which's displays you can use. (max 13 by default)
Select one and you are done :3
How it works:
The script looks about the same as this one here:
Difference to video:
There are no costs.
There are no item restrictions or limitations, except that the items must have the same inventory type.
Transmogrifications are applied by a timed event, not on equip (no hook)
Display source item wont be affected in any way (not made soulbound etc.)