I figured this out a while ago and it took me a long time to figure it out because I was trying all the wrong things.
To do this you'll need
1) A hex editor (I use XVI32)
2) For the ease of things Glitchy's Model Swapper
3) Basic knowledge of both programs.
Lets get started!
Step 1) Open Glitchy's Model Swapper and swap your shield to an offhand or another weapon, here I swap my Bastion of Resolve to Ironmender (Make sure your sheathe is set to 0 for offhands, or 3 for weapons.)
Step 2) Open two copies of your hex editor and load itemcache.wdb on both.(WoW Directory > Cache > WDB > enUS(This may be different on yours due to country) > itemcache.wdb)
Step 3) Use the find tool (CTRL + F to locate both your items)
Step 4) Begin here (Highlighted area) It's the 21st box after the name of your item
Step 5) Since I'm switching Bastion of Resolve to Ironmender, I'm copy from the bottom hex editor ontop of the top one, so copy until you've gotten to the highlighted area (41st Box)
They now match, now save the top copy and exit both hex editors.
Step 6) Login to WoW, you should still see your shield un-sheathed and on your arm, use something that changes your appearance (Deviate Delight, Iron boot flask, or fly out to brunnhildar village in the storm peaks)
Step 7) Click off the appearance buff and hit z (Unsheathe hotkey, you should now see Ironmender instead of Bastion of Resolve, or whichever shield you swapped to whichever weapon/offhand)
Enjoy!