Depends
My understanding is that the Catalina supplied replacements require unstepping the mast since you need to replace the brackets and the compression tube (or is it tubes?) inside the mast. On the other hand JSI can supply replacement aluminum spreaders made to fit the existing brackets. This can be done with the mast in place. You have to send them your old spreaders to use as a pattern. Although I've heard it's not necessary (assuming the boat is going to stay at the dock while the spreaders have been removed), when I removed my wooden ones for repainting (they were in good condition) I replaced them with temporary ones made from a 2x4 planed down to fit in the brackets. I think you'll find more info in the archives.