Switching from Active Duty to Army Reserve?

No you don't shorten it. You transfer/enlist into the reserves the day following your ETS.

SGT (Join to see)
First question: Can you shorten your active duty time, not likely, the active contract will need to run it's course, you can discuss with your retention NCO but they have ZERO incentive to help you LEAVE active duty. If you have someone you can trust and they are willing to provide contacts, then go for it. You can also use the school house for your MOS to get contacts in your targeted reserve or guard units in the state you are returning to join.
For the state it should be as easy as looking on the web for the State on google: 'Ohio national guard recruiter'. The Army Reserve is more difficult,
https://www.goarmy.com/reserve.html
You need to discuss your options with the NG and Reserve recruiter for the state you intend to reside. You may also consider the Air Guard for the state, some will accept cross over.
Good luck and God Speed.

SGT (Join to see) - Ok, lots of questions, no you cannot end you’re active duty contract early, to go reserve. Yes, it is reasonably easy, to re-class MOS. However, you have to be completely ETS’ed from active duty, and in processed, into your reserve unit. For them to send you to the re-class AIT. Pros and Cons, unless you are currently on reserve active duty orders or you have applied for, and been hired into the AGR Active duty reserve component. You will no longer receive an active duty paycheck, your pay will consist of, four days pay, per one weekend a month. You also, would have to start paying for your tri-care coverage, which cost is determined by grade/rank. Keep in mind, you have to qualify for the MOS you wish to be reclassified into, at the time you enlist, into the reserves. Pros, you have more control over your career, and as long as, you’re not due to be deployed, you can switch units, pretty much anytime.