<pulls on numberplate spotter's anorak>
Seniority is the simple answer, insty, but, as always, this isn't an absolute rule (if it was, the GR4 OCU would still be 45 Squadron, not XV).
The most up-to-date figures we have for squadron seniority are from 1968 (tho' I think that another 'which are our senior units?' ex was carried out in 1972-74, which means that the papers might be in the PRO soon), so to work out the most senior plates now means taking the 1968 list, adding on the years of service since then and working out the order - but this is imprecise.
As a for instance, if the GR9 migration to 2 x RAF and 2x RN units goes ahead as planned, one of the RAF GR 7 plates will go. However, as all three are so senior, the plate of the unit that goes will almost certainly be reused for a Typhoon sqn.
Using the 1968 plus service since criteria, 3 Sqn is the most senior of the units, so I suspect the decision will come down to whether 1(F) should be the first front-line Typhoon unit or retain its links with the Harrier.
As for the rest - difficult to tell... Why? Well:
120 and 617 are special cases - they were awarded their standards ahead of schedule in recognition of their WW2 record, and have been treated as being 'undisbandable' [ugh!] ever since.
The next factor is convenience. It may be that the decision is taken to reduce the number of aircraft at a certain base, rather than just chop one sqn from a fleet. So, say, if it is decided that Lossie is to lose a GR 4 unit, it would probably be the case that 12 would go, only for the plate to supersede XV as the OCU plate. If, on t'other hand, it was decided that the GR4 fleet was to lose a unit, base immaterial, 31 (I think) would go as the most junior plate of the GR4 squadrons (617 special exemption kicks in here).
This means that until we know exactly what the planned cuts are, we can't say which for certain which units are likely to disappear and/or re-emerge.
You've also got to consider the fact that their airships have been known to save more junior units because they've served on them, and older plates get the chop instead...
Renumbering is regarded as a bit of a no-no after the farce that went on post-1957, where 3 Sqn kept re- emerging it went from flying Hunters to Javelins to Canberras in the space of 2 or three years, simply because the plate was transferred to less senior units.
An answer might be to abandon the traditional concept of what a squadron is, do admin, etc, via a wing/station system and simply rename each flight as a squadron. Sq Ldrs could command squadrons again...