The concern here is that the top assemble of a threadless headset isn't meant to be positioned on the threaded section of the steerer. So as Fast Jake said, a replacement fork will be needed.

This makes me ask why does the OP want to change his bike's headset? Is there a problem he's trying to fix? he says he's happy with his 600 headset so, again. what's driving this want of a different one?

I sure hope it's not to then be able to use a threadless stem. because that's where the danger will start. The threaded portion of the steerer is full of stress risers that reduce the strength and increase the probability of the stem breaking off the top of the steerer. With a quill stem the quill extends below this threaded section (of the steerer) and much of the stress is then contained by the unthreaded section. Andy.