My understanding is (after asking the chap from the afrmed forces pensions society) there can legally be no retrospection. Any changes can only be made to future pensions provision, what you have already earned, you keep.
Can you honestly imagine the tories taking away pensions away from the forces? I cant. They may pare back 1* and above, or set a maximum pension, but would this necessarily be a bad thing? We might actually get air rank with balls as opposed to staying in the job to get a bigger pension.