Going from active duy to the reserves?

Basic answer, yes. Most reservists come from active duty.

You don't mention which branch, but the Air Force use to have a program called Palace Chase - you could get off active duty early as long as you agreed to serve in the AF Reserves. For every month you got off early, you had to serve 2 months in the reserves.

You can only do this after your Active Duty contract is up. If you are referring to the Army, they also have Active Guard Reserves and National Guard Active. They are pretty much the same, only you will be in your hometown working. Whatever you do, stick with the Military. You will not find a better rewarding place after retirement.

Uh..in any given Reserve or Guard unit at least 20-30% of all people are prior active duty people...

At the end of your enlistment yes. Before you ets, you want to contact a recruiter. As long as you do not have a bar to reenlist, you should be good. Also your dd-214 should have an re (reenlistment code) of 1 or 2. If it is 3 or 4, no luck.

If you are looking to getting out early, do not get out any early than 36 months. If you serve less than 36 months, you will loose lots of gi bill money.

only after your contract is up.

but you can go from reserve to active.