The only way I know is to have the right tools and hope for the best. Once the screw head breaks off, you have one opportunity to try and dill through the screw shank without the drill wandering to the side. You need to use a brand new bit about half the diameter of the srew. Once you have that hole drilled, you can progressively enlarge it so as to take the correct size tap.

Here is where it really gets tough. If you break the tap, you are dead in the water.

If things realy go south, you will have to fit hatch covers and use through bolts. That isn't entirely bad, just not as neat.

BTW, you asked about swapping back and forth...once you have the new pintles, it will work fine.

You saw my old style rudders mounted on the new style pintles.

Good luck Dave.