It has happened many times down the years although the last lot were the first (as far as I know) to have an actual type mentioned on their contract. For the BA internal bidding those guys are not type frozen because they were directed onto a new fleet, albeit on day one, so are free to bid for LH BUT are still at BA's mercy till they have done 5 years. Subtle difference that may see them released before others that are still under a 4 year type freeze.
In the past pilots have been sitting in the 757 sim about to do their final check to be told "it's all changed chaps you are now 737."
Very important that people realise that for 5 years you can be put where you are needed and that you are not part of the Postings process, although you will get a bid. However once out of your 5 year freeze then your posting bid takes prority over any external applicant. BA can't deny a 747 place to an internal bidder and then put a DEP on that fleet. In the next few years most of the Cadets in LGW will be released from their initial 5 year freeze which means that no external pilots will get a sniff of the 747, although the 777 is very unpopular so expect that to always be availiable. However a fleets popularity can change over night. When the -400 first came in nobody would go near it and some very junior guys got comands, that soon changed as people realised how much the long range payments were worth.
But don't take my word for it........or anything off Pprune.....contact BALPA and get the facts for yourself. If you are not a BALPA member then shame on you. I shouldn't bother asking the admin staff at BA. They are about as useful as a chocolate teapot, as those who are about to join will find out.
When the phone goes and you are offered a fleet my strong advice would be to ask which starts first.