I've decided to get a 15 tooth front sprocket for my 400EX and I want to know if i will need a to get a new chain for it or if i can loosen my chain up to fit it... just want to know before i order thanx

400EX come with 15 tooth sprocket stock,
so if you go to 14 tooth, should have no problem.
If going up to 16 tooth, not sure.

rayz400ex
14t sprocket, s-bend, renthal bars
Honda 400EX RIDERS site:
http://home.elp.rr.com/hipo/atv/

I had a slight problem locating your question since it was so much shorter than your sig, but I finally found it
The first thing I'd do would be to check your chain and present sprockets for wear. If they show no wear then you can use the present chain. Check the teeth on each sprocket for signs of hooking; the points of the teeth should be straight with no hooks at the tips. If hooked, it is due to a worn out chain & its use on new sprockets will ruin the new ones quickly. The reverse is also true; that is, if you put a new chain on a hooked sprocket, the chain will be history.
To check for wear on a chain, adjust the chain to specs then grasp a link at 3 o'clock on the rear sprocket and try to pull the link away from the tooth. If you can pull the link away from the tooth more than 1/2 a tooth, the chain is shot.
I run a 15 tooth c/s on my Warrior and use the stock length chain. It was a bit tricky getting it over the sprocket but once I got it on I still had enough slack.