I switched my finesse G4 cups awhile back, and just switched them back to the originals the other day. Apparently not all colours can be switched, and I have no idea the logic behind it, but its true. I have a white one and an orange one, and they're very slightly wobbily when the cups are switched. I thought they were wobbily because of just excessive use, but when I switched them back, they spin very smoothly.
If you want to give it a try, you need an alan key/hexwrench. I'm not sure the exact size, but if you buy a kit, or bring your diabolo to a hardware store, you can find the right one. One of the diabolo cups has a long screw, and the other one has a nut that the screw goes into. Make sure you're not trying to put the 2 screw ends together, or the 2 nut ends together. Take them apart completely, but keep them seperate, and just switch the matching pieces from the cup you want to switch.
Hope this helps.
