I don't think you can realistically transition to the reserves after 20+ years of active component service.....
Anything over 17 ish years would be a no-go, especially if the member is close to sanctuary.
I'm not sure about the reserves, but there is a waiver for everything in the Guard. Though that waiver requires specific circumstances. For instance, if you are going to take command of a squadron or group and stay a drilling status guardsman, or work in a staff job as an O-6 on an active duty tour, or on the Adjutant General's staff, or you get hired into a full time Air Reserve Technician slot where you have to be a drilling member of the unit.
But most places would rather promote from within to that level than take someone off of AD. The stars would have to align, the timing perfect, and you'd have to be willing to do a job that no one else not on AD is qualified, or willing, to do.