Transmogrification is a feature coming in patch 4.3 (hopefully) which will allow you to change the appearance of a current piece of armor or weapon that you have equipped.
Posted on the US Forums 19/9/2011
"Since we announced the transmogrification feature coming to World of Warcraft with patch 4.3, we have monitored thousands of follow-up comments, questions and suggestions from players all over the world.
What follows is our current list of rules for transmogrification. This is a set of rules that we expect to change over time, and is not intended to be 100% comprehensive, but we hope that it can be referenced to answer most questions that players currently have about what they can and cannot expect to be able to do with transmogrification.
- The character must be able to equip both items.
- Only uncommon (green), rare (blue) or epic (purple) items may be transmogrified. (exceptions: a select few of these items will prevented from being used to transmogrify if they are inappropriate)
- Items must share the same armor type (examples: plate for plate, cloth for cloth)
- Weapons must be the same weapon type (exceptions: Guns, Crossbows, or Bows)
- Guns, Crossbows, and Bows can be used to transmogrify Guns, Crossbows, or Bows.
- Main hand weapons can only be used to transmogrify Main hand weapons.
- Off-hand weapons can only be used to transmogrify Off-hand weapons.
- One handed weapons can be used to transmogrify a Main hand or Off-hand weapon.
- Using an item for transmogrify makes it soulbound.
- Using an item for transmogrify makes it non-refundable.
- Using an item for transmogrify makes it non-tradable.
- Heirlooms and Account Bound items can be transmogrified.
- Heirlooms and Account Bound items can be used to transmogrify.
- Legendary items cannot be transmogrified.
- Legendary items cannot be used to transmogrify.
- Fishing Poles cannot be transmogrified.
- Fishing Poles cannot be used to transmogrify.
- Mailing an item strips its transmogrification.
- Placing an item in Void Storage strips its transmogrification.
- Vendoring an item strips its transmogrification.
- The displayed enchant will be that of the currently equipped item.
- There may be individual items that are excluded from being transmogrified on the basis that they were originally added to the game as absurdities. (examples: a weapon that looks like a fish, or a chest piece that is invisible)"