I helped my dad do some many years back, and then helped a friend fix some up about 8 or 9 years ago. It was as simple as unwinding the yo-yo all the way, clpping it, and tying on the new line, then slowly let it wind the line back in. Makes it a bit easier with two sets of hands, but is not difficult.
Thanks for the replies. I tried to pull the line to the end the other day on a couple. Seemed the spring would get too tight before I could get all the way to the end. I'll give it another shot when I get a chance.