That 'chance' might become enforced, as automatic service to 55 for Sqn Ldrs is pretty much guaranteed to disappear in the next few years, IMHO. Some will be 'assimilated' at age 40 but I would expect others to get the boot at that point.
Not entirely sure how that one will work if people are already serving on T&Cs that guarantee them employment to 55 without some sort of payoff. Unless you make future promotees sign up to the principle of up or out by 40 when they accept promotion to sqn ldr, I think that any half decent employment lawyer should be able to challenge such a change to existing T&C.
That said, there will probably be quite a number where the idea of stagnating in rank / pay for 15-20 year isn't appealing and they go of their own volition, so 'bed blocking' might not be an issue. The issue, however, will be how to retain those individuals with critical skill sets and Qs, QWISR for example given the rapidly rising importance of ISR, or those in the more specialist branches which even on a good year don't have rapid promotion. In those cases, an up or out policy might well prove detrimental to the overall capabilities of the service.