You'll have to heat up the plastic cups a bit to soften the glue. Once that has been done, you can carefully separate the plastic outer cups from the plastic piece holding the driver. I've done it before with my heat gun. I carefully turn the cups as I'm applying heat. They have to get pretty warm in order to melt the glue. So, ensure you have some rags handy as you'll need the rags to cover the plastic pieces when you grab them to twist open the cups. There's another method that has been documented which involves a shallow pan of water heated in the oven. With this method, steam softens the glue so you can pull them apart. I've never done the steam method before, just the heat gun method. If you don't have a heat gun, you can use a hair dryer. Keep the cups turning while you're applying heat as you don't want to melt them. But, like I said, you have to get them pretty warm.
Typically, Grado just glues their plastic cup pieces together with glue from what appears to be a hot glue gun. When you re-assemble them, you could use a bit more hot glue, then quickly push the cups together and let the glue harden.
It's not a bad process. Just go slow and you'll be OK.