For most of them, you just have to pull out the axle and the sheaves will slide out forward/backward. The problem with doing them in place is what do you use to haul yourself up, maybe a spinnaker halyard
If you are switching from wire to rope
halyards you don't need to change the sheaves unless. If the sheaves are rope/halyard they will work just fine. Have sailed more than 12,000 miles with wire/rope sheaves and rope
halyards. Might want to inspect the sheaves, in any case, just to be sure that the wire hasn't created any sharp edges that could cut the rope. Hasn't done it on either of my boats, however.