If a 3rd F-35B sqn is in the longer-term offing, it seems fairly likely that it would get a FAA numberplate, which means that options for keeping the IX(B) Sqn numberplate alive post-Tornado OSD are few. As a single-digit number with quite extensive senior backing (VCDS and DCom Ops are both former commanders) I suspect that one of the rumoured new Typhoon sqns will take the numberplate, either on disbandment of the third Tornado sqn or at Tornado OSD. Protestations about fighter heritage should be given a stiff ignoring given that Typhoon will shoulder the majority of the RAF's attack duties (with Storm Shadow, Brimstone 2 and Paveway IV) even after F-35 enters service - and let's face it, it's far more likely to use any of those weapons in anger than it is to shoot anything down. I reckon the Air Force Board will take the view that 'hibernating' IX(B) for a potential 4th F-35 squadron is too risky an approach.
31 and 12 Sqns? Looks like the history books beckon, sadly.