From memory the advice on RCGroups and elsewhere is to glue them back in with some epoxy, using a toothpick to spread it around the can evenly so as not to upset the balance too much.
Quote from an AXN build wiki:
"Some motors might develop a problem with the magnets shifting from their position. Some magnets might have insufficient Epoxy glue to hold them in place over repeated use. Do an inspection and if necessary add small amounts of clear Epoxy glue into the slots around the entire barrel. This is to keep the mass when spinning around the center balanced. "
Full wiki found here:
http://beta.ivc.no/wiki/index.php/AXN_Clouds_Fly_Setup#Motor
What goes up...better bloody stay up (until I say otherwise).