How do you transfer to the Army/Air National Guard or Navy Reserve as an E-8 or E-9 with 20+ years of reserve service?

I was Marine Corps Reserve for 18 years (SSgt) and purposely switched to the army national guard for that specific reason. I wasn't done serving so switching branches was the only option. I went to OCS and commissioned as an O1E with the availability to serve an additional 20 years to my retirement age. I know this doesn't specifically address your question. However, serving past 30 will only be possible in the army/air guard.

Well I seen it done. I was just active with a SFC (E7) who was in the corps as a GySgt with over 15 years in the corps. He switched over a few years back to the Army reserves, he just got sanctioned at the end of last year because he ended up having over 18 years total of Active duty time so they forced him to stay another 2 years to retire on Active Duty. I would say give a shout out to a Retention NCO in the Army Reserves and he would be able to help you out with everything.

I might suggest contacting your local recruiters for those branches as well as the local Manpower/Retention folks for the appropriate Reserve components. Check out your options. There may be Officer conversion programs available to you or other such things if you are willing and otherwise eligible. Can't hurt to shop around. Of course, remember to get it all in writing! Good Luck!