95 Buick LeSabre A/C Clutch Replacement

I have a fairly new A/C Compressor (just out of warranty) that has a bad coil and I have an old compressor that has a good coil. Unfortunately, I can't figure out how to get the clutch off of the HD-6 Compressor. The compressor has a threaded shaft with a keyway, but it is not clear if the key needs to be removed and the shaft removed with a special tool or if the hub is just a press fit and the key is just for alighment purposes. Can anyone please help?