The only idea I have is trying to break off the part of the loop end that keeps the loop shaped (I think it's called a thimble, maybe?), so I would instead have just a loop of wire. I could then squeeze the loop end so that it would fit through the pulley system and come out the other side.
You've more-or-less come up with the proper way to do this.
Cut off the eye as close to the crimping sleeve as you can. Remove from old mast. Install in new mast. Put in a new loop (without the thimble) so you can tie the line portion of the halyard to it, and make the change in the future without further shortening the wire halyard.