Did this or something like it on my Ranger 29. Different boat of course. I agree the mast boom end has to come off and the boom needs to be set up to work on - like on saw horses. The block on the mast end needs to come out of the boom. It is attached to the boom in some way: Rivets, machine screws or, God forbid, bolts. Rivets get drilled out, machine screws backed out (Unless corrosion prevents) and bolts ... Hopefully not the case.
As long as the tail is long enough, you pull that whole block out of the mast end of the boom. The block that is in the aft end of the boom has to travel forward so that you can pull the mast end one out. So you have to rig a messenger line to either the block or the shackle that attaches to the sail.
Once the mast end block is out of the boom, it will be fairly easy to untie the knot. attach the new line to the old line (Cutting the line past the knot which will not go around the blocks easily), and reeve the new line through the blocks and eventually out the mast side of the boom. Tie the new line to that block. Once that is accomplished you can use the messenger line on the sail shackle or aft block to pull the mast side block back into the boom.
You then have to reattach that block to the boom. No one says it has to be in the same location. So pick a spot you can reach with your hand or some implement. Reattach the boom end.
That is how I would approach it based on my understanding of your drawing. It is possible that the mast side block is on a pendant, which would make it easier.
If your reefing lines are also in the boom caution is needed to not tangle them. Reeving a new line using the old on should prevent entanglements.
For me having the sail deepen in gusts was annoying. This would induce more heel, load up the rudder, increase weather helm and ultimately slow the boat. With a proper tight, very low stretch outhaul in a gust the boat will dip its shoulder and come up with acceleration - big difference. I changed the outhaul line on my H356 (With in mast furling) for the same reason and even better results. I don't regard this improvement as trivial.