Gearing for DRZ400s

I bought a new 400 and now have 500 miles on it. The gearing is great fro the road and OK for fire roads but in any kind of tight stuff it seems way too tall. What I would like to do is just change the front sprocket when I intend to do some serious trail riding but change back to the original gearing for general riding.

Has anyone done this? How many teeth can I drop and still use the same chain? How much difference does it make?