I did mine w/o going up the mast.
It can be done w/o going up the mast but seemingly no one recommends it. On my 1984 Catalina 30 I was able to undo the headstay, remove the roller furler, reassemble it with a new halyard and reinstall it on the loosened headstay. I just made sure the lower stays and shrouds were tight, loosened the backstay slightly, ran the the mainsail and the spinnaker halyards tied to the bow rail to support the mast. Then I slacked off the headstay turnbuckle a bit, disconnected the headstay, did the work and put everything back the way it was. Worked fine for me even though there was a mild breeze at the time. Everyone I talked to said not to do it that way. Previous experience with the boat made me take the chance. Took me about 2 hours and had no problems. But, of course, if anyone wishes to do it that way that would be their own decision.