Not possible unless you are willing to be very creative and take some risks. Maybe a bettter job for a profesisonal rigger.
Because the boat is fractional, the only way to reach the masthead is via the main or topping lift halyards. The trick is those two sheaves share a common axle, and you cannot unload only one. So you would have to go up, transfer your suspended load to another secure point, and do the work.
You might want to take a quick trip up anyway. Unless someone can tell you definitively, you want to see if the sheaves are mounted in a way where they can be replaced in that orientation. Some are really set into the spar.