When we first started replacing the old wire, installed in 1988, the sheaves were indeed often a bit narrow for passing the fatter line. i.e. if you had wire spliced to 7/16 or 1/2" line tails, that sheave at the top of the mast might have been too narrow for new line.
Later, much stronger line came onto the market at more affordable prices.
For instance, our 34 footer, delivered with wire-to-rope halyards, has masthead sheaves that will pass 3/8" line with some friction, but work perfect with 5/16".
And since the 5/16" will hold fine in the original clutches and the housetop ST winches, the perfect solution was and is some T-900
hi tech line.
One other caveat -- after many decades of abrasion from the wire, your masthead might need to have the sheaves removed and turned (if alum.) or just replaced if the axles and sheaves are about worn out. That's a separate but needed part of long-term maintenance on any boat.
After getting bloodied on meat hooks on old wire, you'll love the changeover to all-line. After all, the only reason that wire was ever used was to get around the stretch in the older line materials.