I've opened a bunch of SW speedos using a slightly modified paint can opener, they have a loop handle and a sort of 90 degree crow's foot bend at the bottom. Years ago the hardware store used to give them away when you bought paint, now you have to pay for them but they're still pretty cheap. You can file the "foot" so its a little thinner and shorter in the "L" section, then just work it under the bezel lip and walk it around like Cotten described. If you do it slowly and carefully you can straighten the edge without creasing it.
Another cheap but essential tool for working on the old speedo is a block of wood with a circular hole recessed into it to hold the unit while you work on it. The wood block is even more important when you are trying to crimp the bezel back on, extra important if you're working on one of the old ones with domed glass. I also use an old hammer handle with an arch shaved on one end the same radius as the bezel. Be patient, work slowly, a little bit at a time, and you can "press" the bezel back on so it looks almost factory sealed.