We worried about changing our wire/rope halyards to all line too. Are the sheaves, sized for wire, too tight a turn for the new techno wonderstuff? It appears that wire needs a bigger radius than what''s now available. This is therefore a nonissue.
Are the old sheaves going to chew up your expensive new halyards because of rough edges caused by the steel wire abrading the softer aluminum or composite sheaves? Do they abrade the rope part of your current wire/rope halyards? Get some sandpaper and smooth them down. Two sheets at $0.75 each : $1.50.
Is your new halyard going to jam in the sheave because it''s been designed with a wide part for the line to run in and a narrower slot for the wire to ride on. As Sasha V suggests, get a new line of sufficient diameter to be comfortable to hold, and it won''t slip into the tighter slot.
We changed out our wire/rope spinnaker halyards to all line five seasons ago. The meathooks were getting excessive, and we learned that an all-line halyard would actually stretch LESS than wire. A single strand of some of the stuff available, say 10 to 12mm, is strong enough to lift a 10 ton keelboat. We were concerned about all the above issues, but have not had ANY of the problems described. We''ve examined our halyards carefully at the end of each season, and the only problem seems to be the sun fading the color on the exposed parts.
The best idea might be to simply use all line and examine the halyard and sheave at the end of the season when you pull the mast. If the sheave should be changed, you''ll be able to tell. If not, you''ve saved $20 for a new sheave, and perhaps $150 for a wire/rope splice. Of course, there are the purist restorers who want everything to be "period", but they sail with reel winches for their all-wire main halyards and have an orthopedist on retainer for when the brake is released with the handle in.