If you drop in chain size from what the factory size is expect to be adjust the chain abit more than you are now. That is unless you pony up the cash for a NICE chain. With my dirtbikes I would kill a DID in under a season. The best I found for not stretching is a Tsubaki chain.
U.S. Tsubaki
I liked the Regina chain as well, it lasted a while but the gold finish wore off after one run.
Some good sprokets are Renthal, Sunstar and sproket specialties. (Sproket Specialties will cut you any sproket you desire if you cant find the number of teeth you are looking for.)
Also, Al sprokets are nice however they will tend to cup on you after a while so you may want to stick to steel.
As for a write up on changing out the sprokets. My advise would be as follows:
Remove old chain by removing the masterlink and pulling the chain off.
Remove rear wheel. Apply lots of PB blaster on the nuts that hold the rear spoket to the rubber damper on the rear wheel. Let sit and apply again. Using a
closed end wrench remove the nuts and then remove the sproket and appy the new one. When putting the nuts back on, apply thread locker.
Counter shaft sprocket: Remove the cover and using a pair of C-clip pliers remove the C-clip and slide old sproket off and then add the new one, re insert C-clip.
The chain. Here comes the fun part.
First put the rear wheel back on and run the adjusters all the way in so that the axle is a far foward as possible.
When you order a chain to your specified length, the always come a few links too long. (Always) You will need to cut the chain to length. Now you can do this with a pair of chain breakers or you can use a dremel and grind down the sides of the link so that you can take it apart to the desired length. From there instal the new masterlink and you are good to go as soon and you double check things and make sure that you axle is straight and tight.
Also just as a side note if you want a neat way to see if you chain is shot. Remove the chain and lay it flat on the floor. Then take the two ends and move them towards you in a bending motion. The chain should curve in some, but if it moves in to a nice cresant shape, its toast.
I had a DID after one season almost make a U!
Also I like to service my chains from time to time by taking them off and soaking them in K-1 and scrubbing them till they are spotless. This will also remove a majority of the oil out of the chain so from there I let the chain soak in old engine oil for several days then hang it up over the pan from one end to drain off the excess.
Once it is back on the bike I coat it in my favorite chain lube.
-bryan