Use Brain :)

push clips in and profit

Use a flat screwdriver and wedge it in between the clip and the shell.
Turn the screwdriver forcing the clip back.
One its touching the opposite wall of the shell, gently push the clip down into the hole.

It should snap open. Repeat for the other side.

Note: Be very careful with these as the plastic is not very flexible and those clips are very uasy to break.

Thanks. I left it for now, going to wait until I have a spare WMO to try again because it does seem easy to break them. I was pushing the clips back and trying to pull the shell off, wasn't working. But doing what you said sounds like it will work.

I broke my first WMO whilst trying to do this. As 2shell said, the plastic is VERY delicate.

NB: The new gen of the WMO has soldered in wires, so you cannot steal the wire from it for other mice with minimal effort. Mice like the intellimouse 3.0 and the 518 have clipped cables which means that they are theoretically replaceable.