Once you enter sanctuary, even on the first day, you declare your right to extend until the 20-yr point. What they can do is send you wherever they want to, big blue owns you at that point (i.e. there is no obligation to honor any prior LPRP promises). What they will do depends on how long you have until 20 and what you are currently doing and where you currently are; that part takes a crystal ball. I don't think the USAF has a habit of purposefully slamming guys just because they made it to sanctuary.