Actually it turns out theres an easier way. Roland at one time made the BC-13 bus converter which was an adapter that converts 24 pin signal (the old roland cable type) to the newer 13 pin signal so that you can drive newer guitar synths. This would preclude the need to replace the GK1 pickup. However the BC13 is just shy of extinct. So who knows if any of us will ever be able to find one.
The only other reason I could see for replacing the GK1 with a GK2A would be if there is some sort of performance difference b/n the two, and I'm betting there isn't.