Yes, the continuation rules are set for each board, and they've been expanding the pool recently. My 2nd passover happened in 2006, and continuation was to 20 for everyone. Back around the 99-2000 time frame they were offering continuation to 24 for all line Majors. We may be coming full circle again.
I had thought that my initial continuation offer (after 2nd non-promotion) was final. But last year I was offered continuation to 24 after my 4th passover because I had one of those select few AFSCs (11S, special ops). If what you say is true that they've expanded the 24 year offer to more AFSCs, then it's a good bet that they may offer it to all pilots soon...esp in light of the return to active duty offers and such.
A couple things to note: accepting continuation to 24 does not commit you to 24, it's not an ADSC - it just allows you to serve beyond 20. Second note: there is no requirement to accept the 24 year offer, you can decline continuation to 24 and stay with your original offer to 20.
Third note (or maybe this should have been first): BY LAW, a DOS established for non-promotion trumps any ADSCs. That's why Kikuchiyo's bud had to retire at 20 instead of finishing his ADSC. When this happens though, if you are on an ADSC for a bonus, and if you took the lump sum option, you will probably have to pay back some of that lump sum, prorated for the amount of ADSC you do not fulfill. I'm not completely sure on that though...since it's a mandatory DOS you might be ok.
When the next O-5 board results are released, and if they are offering you continuation to 24, they will send you a couple documents spelling out all of the options and ramifications. If there's no offer to 24, then your 20 year DOS is mandatory and you won't have to finish that ADSC to 21. I'm certain of that. The rest of it, like whether you have to finish the ADSC to 21 if they offer you 24, I'm not quite sure. I do remember that in the continuation documents, I could decline the offer to 24 and keep my DOS at the 20 year point.
I hope I helped a little bit, I realize I may have confused the issue.