No problem.

Sticking the Magnum driver into an SR60i is a bit harder, but not impossible.

The biggest obstacle is liberating the Grado driver from the plastic sleeve without breaking the plastic. I've seen someone use a clamp and wooden block to do it, so it can be done. The other thing to consider is the Magnum drivers seem ever so slightly smaller than the Grado drivers. So, you'd have to use something like electrical tape to fill in that gap (between the driver and plastic sleeve). But if you can swing it, at $200 that's be fantastic.

The semi-full-Magnum I was thinking of is to buy the driver+sleeves, and then it's just a matter of placing them inside the outer cups -- but that would end up costing about $320. From what I understand, the drivers were tuned with the aluminum sleeve in mind -- and would result in less resonance.