I need to either replace the sheaves at the top of the mast or lubricate them. (loud squeal when hauling up the main)

Something of a catch-22 as I wonder how strong they are at this old age.

I figure I can change the halyards by sending up a messenger and pulling up a new halyard... so I know that will be good... but can one change a sheave while aloft? Obviously you work on the sheave that is not under your load... as I have 2 halyards I would work on "the other one."

This limits my safety line though... is this even safe or possible to do?

Or do I just do it in a yard?