Automatic, retail limitations, all equippable items, Trinitycore 3.3.5a tested, works with custom items automatically
The script is done with gossip menus
The items are not hardcoded, only the limitations and whatnot are in the script.
Limitations:
So you can only transmogrify epic, uncommon and rare items.
You can also only change the displays with same type weapons only etc.
You can not try getting a level 80 looking gear with level 1 char.
Check out the full list of requirements from the video above, thats where I got them.
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 etc. somewhere in your inventory (doesnt matter, just in some bag).
When talking to the NPC you get to select a slot where you have an item equipped.
After selecting you see a menu with item names of the items which's displays you can use.
Select one and you are done