Like others have said, if the change wasn't made clear when they changed the rules, then you have a case. If they have again changed the rules, back to where they had been previously, I would have thought you have a case as it almost looks like they are discriminating against you - they must...
Ah, good old terms and conditions. Whilst it may not have been specified on the initial paperwork and application form, I would imagine that there will be something in the policy conditions about having an animal checked out at periodic (yearly) intervals. If it isn't in the original application...