A good question and normally answered in the sim by, "it's probably an airfield thing and ATC will advise that they're now only Cat 2 capable".
Some research revealed the following unlikely additional scenario applicable in Canada:
Reported RVR 1200/600/600. Aircraft starts a Cat 3 approach. Required RVR 600/600/600
Reported RVR then drops to 1200/600/500. Cat 3 no longer possible as all three RVRs are required to be 600 or greater.
Cat 2 approach possible, as required RVR is 1200/600/advisory.