[Arran]

Melting them out is actually easier then you might think, I place about four or more caps at a time, on an aluminum foil lined tray in an old toaster oven, I think set at around 200-250 degrees. I leave the caps  on the tray until the wax plugs start melting, remove each one at a time, then pull the guts out with a pair of pliers whilst holding the body with a rag, then I put each cardboard tube back in the oven on end to let the rest of the wax melt out. You can then go over each one with a heat gun and paper towel to wipe the rest of the wax residue off, and you have a nice, clean, capacitor tube ready for restuffing.
  To restuff I usually wrap the new film capacitor with a strip of thin corrugated cardboard, maybe 1/8'' longer at each end then the body of the new cap. I then stuff the new cap, with the cardboard wrapped around it, about mid way inside the old cardboard tube, then I fill each end with hot melt glue. It helps to hold the whole assembly on end after you fill each end with glue so the glue has a chance to harden before flipping the cap over and fill the other end.
   With regard to slicing the tube open and prying it apart to get the guts out, or crushing the wax in the ends, I'm not sure that it would work without damaging the tube, at least not without heating the tube with a heat gun. You could try it on one you don't have any plans to restuff as an experiment. The caps RCA used to use from the mid 1930s till the early 1940s are really easy to get apart without heat, you can push the capacitor right out of the tube using a dowel or a mechanic's socket, by placing the capacitor on top, grabbing the body, and pushing downward against a bench top or hard surface, the ends on those cap tubes are rolled over cardboard disks.
Regards
Arran