Sheaves are generally NOT easy to change, especially with the mast up, and it may not be necessary to change the sheave anyway. It depends on the sheave you currently have.
If your current sheave has a groove in it for the wire, with a larger diameter section for the line portion, you may want to make sure your new all-rope halyard won''t get jammed into the groove-- make sure the new line''s diameter is too big to fit into it. A line that would fit in the groove could probably still be strong enough, but it would be too thin to pull on easily. If there''s no groove in the sheave, don''t worry about it. The new lines take more kindly to larger diameter sheave sections, and so will probably work fine on the existing sheave. Technically, a flat-channeled sheave is supposed to be the best for the new non-stretch lines, (the channel has a flat bottom instead of a curved one), but this can create problems with the line jumping the sheave, which you do not want, and require redesigning the whole setup with guard bails and guides which you do not want either. We changed both our spinnaker halyards three years ago without switiching sheaves on our J/36, and have had no problems. The K.I.S.S. principle works for us.