Ok, alot of people have been asking for this guide so I'm going to post a guide about how to do this...
Step 1. Open MWCS and open common.mpq
Step 2. Under common.mpq go to Item > ObjectComponents > HEAD (yes with the caps). Under that should be a huge list of weird file names and you are going to find the helmet you want to change into. Here is how it works... Let's take Helm_Plate_D_01_TaM.m2
How that works is that Plate is the type of armor it is hense the word "plate" and the TaM part means Tauren Male. So if your Troll Female, your looking for TrF (for blood elf is Be and night elf its Ne before anyone asks that they can't find those). Also REMEMBER THE NAME OF THE HELMET FILE YOU'LL NEED IT LATER.
Step 3. Extract this helmet file to the desktop.
Step 4. Create a new folder called "Item" on your desktop and in that folder create a file called "HEAD" and then drag that helmet file into the "HEAD" file.
Step 5. Change the last part of the file (the TaM or whatever) to whatever race you want (make sure its like TaM in the 3 letter format).
Step 6. Go back to MWCS and go to Pack > Create MPQ archive. You are going to save this as patch-x.mpq (replace the x with a number between 3-9)
Step 7. Now click in the white area in MWCS and click Import Folder. Now import the Item folder from your desktop into MWCS.
Step 8. Save the mpq file and delete the "HEAD" folder under the ObjectComponents folder under the Item folder on your desktop.
Step 9. In MWCS under common.mpq go to Item>ObjectComponents>Head (without the caps). Look for the EXACT same file that you did before but with .blp at the end under the Head folder. Once you do, extract it to the desktop.
Step 10. Under the Item folder that you made, make another folder called "Head" (WITHOUT THE CAPS) and drag the .blp file into the Head folder.
Step 11. Do the same thing as Step 5 but with the .blp file
Step 12. Go back to MWCS, go to Pack, Create MPQ archive. We are going to save this under the Data folder in the World of Warcraft directory. Name it to patch-x.mpq (replace the x with a number between 3-9)
Step 13. Click on the white area in MWCS and press "import folder" and go to your desktop and load the "Item" folder into MWCS. Now save it and your done.
P.S. You should have 2 different MPQ files by the end.
I take full credit for this guide by the way.