Flaps, I can't see the 5 year "engagement" freeze reducing in the near future as it is entirely in BA's favour; they can essentially do what they will with you for the first five years. Believe me, if it suits the current plan they will grant early bids sometimes well within the initial 5 years. The B-side of this is that they can also direct you to other fleets/bases as they see fit.. this happened to some rather un-gruntled 744 pilots in the last couple of years, and is the subject of ongoing strife with them... fingers crossed lads. Haven't heard of it applied that way in recent years prior to that event.
Outside the 5 years they are fairly strict on the 4-year (previously 3 year) type freeze following conversion course, however if you are bidding for initial command, then subject to the "6 months continous operation on current type" requirement buried in the Annual Bid Document that will be waived. Currently just that is happening on the Airbus at LHR, my sources tell me.
The best advice is bid every year for what you want, not what you "think" you will get. Then, should the vacancies arise, all freezes have a habit of melting (mixed-metaphor alert!)
All the best,
£6