I have seen cans like that held on by tar. Took a heat gun and patience, but the stuff eventually softened up enough to pull off the cardboard.
As a last resort, you can slit the cardboard on one side with a razor blade to ease removal. When reinstalling, glue the tube back together and put the seam on the least visible side.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html