I get the feeling I am arguing as the devil''s advocate here...
If the halyard he has now is double braid that is spliced into cable. Lets say the double-braid is 10mm. You change out the top sheave sot hat you can run 10mm spectra throughout. It gives you something much stronger then you used to have. It is lighter and friendlier to the touch...and it fits all stoppers and drums and ST''s in exactly the same way as the double-braid used to.
he only place you need to replace anything is where the steel cable used to run. And that is just the top sheave.
Really, this IS easy. I have done it, there is nothing to be scared of.