Interesting, and yes, if you change over to all cordage the sheaves should be changed. However, I've found that the smaller diameter 12 strand dyneema or vectran (ie 3/16" diameter for boats of our size), lies snugly in the wire groove of the combo sheaves usually found on our boats. You should, no matter what you do, carefully look at the sheaves and clean them up with a file and emery paper. Decades of wire use will usually leave lots of wear which will quickly abrade any cordage you replace the wire with.
Originally Posted by Boasun
If you change from wire to line for your halyard, you have to change out the sheaves at the top of the mast.
1. With a wire/rope halyard the sheave has a grove in the base of the sheave for the with with a wider one for the rope.
2. With an all line halyard you will want the appropriately sized grove to fit that halyard.