Ok, I've known about this for a while now, and I noticed someone posted how to do a helmet fix, so I thought I would show a way to fix all helms for your desired race -> race edit.
Step 1: Open Up Mywarcraftstudio and Open Locale-enGB.MPQ inside your enGB/enUS or whatever folder.
Step 2: Find and open ChrRaces.dbc in the folder "DBFilesClient" Extract that file to any destination of your choice (you will need this file, so remember where you put it )
Step 3: Open up your DBC Editor, or whatever you use, and open ChrRaces.dbc.
Step 4: This is the editing part, As an example I am using Human -> undead model changing.
Go to field 6 and where it says "Hu" Replace it with "Sc"
Go to Field 12, and where "Human" is written Replace it with "Scourge".
[this part is optional, it makes your character sheet different in wow. eg: if I opened up my char sheet, it would say 70 undead mage instead of 70 human mage]
Go to field 14 and replace "Human" with "Undead".
Step 5: Save it.
Step 6: Go back to Mywarcraftstudio and go to Pack(P) and click Create MPQ archive or Edit MPQ archive, whichever you prefer and Call it patch-1-9 or patch-a-z eg: patch-1 ... patch-2 ... or Patch-a ... patch-b... etc. Then click OK
Step 7: Click Pack(P) again and click on Add File to Archive. find your file, double click and your treepath is DBFilesClient\ChrRaces.dbc
Step 8: Save your MPQ and now hopefully it should be working fine.
As for Changing the Sex Helms ie: Human male -> Human female, I don't know how to do that, but maybe someone can get something from this to do it.
~~ Karakanz