30MM,

You need to look in AFI36-2110 Chapter 3 on Overseas Duty. A few months ago I tried to go into the seperations office at Ramstein to start the paperwork and was told I couldn't yet because my voluntarily extended DEROS was my new "ADSC". While my case is a little different than yours I seem to remember some verbiage that basically said what the MPF weenie said, the AF treats your DEROS as an ADSC. You can request to "curtail" it, but I think you will probably have to get your wing CC to sign off on it and then it goes to AFPC for final approval. Reference table 3.11 Rule 3 and 4 if you are curtailing it for an approved seperation. It either requires your weapons system manager at AFPC (if date is less than 60 days) or HQ AFPC/CC (greater than 60).
My situation was quite frustrating since my DEROS extension was a drug deal where I didn't sign anything. My AFPC assignment manager has been easy to work with since it was a voluntary extension . I should see my DEROS changed next week so I can start my seperation paperwork.
Not trying to rain on your parade but it may be an uphill battle.