Got this from another site:
02-08 will interchange.
06 is the best of the 02-06 heads with the better seat material. All 02-06 heads use the same cam/decompression system.
07-08 heads are the same with the smaller valves than the 02-06 heads.
07 is a 1 year ONLY cam.
08 is a 1 year only cam!
both use the weight style decompression system, but the gear and the cams are not compatible between years. 07=07 ONLY parts. 08= 08 ONLY Parts.
Basically you can use any year head , as long as you use the Cam tower for your bike , it should adjust and function the same , the valve size difference will not affect actual settings like valve lash , etc... unless you find a Honda Part # , for example , the part # for a 2007 CRF450R head is 12200-MEN-A00 ....... The part # for a 2003 Honda CRF450R head is 12200-MEB-770 .......... You can find that info by locating the part # on the head itself (I can't remember where it is located on the head) and looking at the Parts Fiche and matching the number to each year till you find it http://fiche.worldofpowersports.com/yel ... ke=hondadb
The water outlet was changed from the thin chrome one to the thicker Aluminum one , though I have never had any issues with the thin one , so not sure what the significance of the change is for , maybe , corrosion or something ..... , but basically you need to locate the part # of the head , and go on the parts fiche and find what year matches that number (may be several years) , and then just get parts for that year .... but the gaskets themselves should fit 02-08 as far as I know ...