Called a "topping lift"
What you refer to is called the topping lift, as I understand it, on hte O'Day 25 it was rigged as a line that was "deadended" at or near the masthead (tied or spliced to an eyestrap) and the adjustment was made at the clew end (outer end) of the boom. Does replacing it require going up the mast, well no, not if you do it while the mast is unstepped as you should do at least every other year if not every year at haulout time. I suppose you could send someone up the mast to attach the new line, but I really would not be comfortable hoisting someone up the mast on a 25' boat especially if your 25 is a K/CB model. It is a lot safer to unstep the mast. I'd recommend giving Rudy Nickerson at D&R Marine a call for advice, he will know the best way to replace the topping lift and can supply you with a ready-made replacement. D&R Marine (508) 644-3001 or check the website link below.