Every item in the itemtype.dat has two entries. The first one will be close to the top of the file the second will be further down. In this example I will use the normal BlueCrestHelmet
Step 1 would be to find the id of the helmet and search the itemtype.dat starting at the first line. In this example it is 111000 which in hex is 1b198 which then you have to reverse the numbers so it turns out to be 98 b1 01
So do a search for 98 b1 01 and you will find the entry as in the following image.
Now you will want to change that id to the one you want. Next you will want to find the first id 98 b1 01 again and you should find it a little further down, example is in the next image.
Once you find the second entry, change that to match the new id you created for the first entry. Then save the itemtype.dat and your done.