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"How does one switch from three different furling headsails? " The same way one reefs a conventional mainsail: before it is needed, or else with a lot of extra effort.
When you have two tracks on the headfoil, traditionally you unroll "this" sail all the way. If necessary, trim to depower the boat and head downwind. Now, you can raise the second headsail (you do have a second halyard of course, and have matched halyards port/starboard to headfoil tracks) inside of the first one. Then drop the first headsail, keeping it inside the rail.
As with reefing the main, the problem comes when you realize that you "shoulda" anticipated the wind change but now are way overpowered. In that case you might want to head upwind, put the boat in irons, and drop the excessive sail from the track. Whatever works out best for you and your boat. Cruisers, after all, have the option of "stopping" to do whatever is best, without worrying about keeping up boatspeed for the race.
Wing and wing is *usually* a slow point of sail, but some boats do well on it. I was trained "no no no" about w&w but finally met a boat where it actually was just as fast as anything else--and the polars confirmed it!
"Polars" refer to polar graphs, usually plotted in terms of boatspeed versus direction versus windspeed, for a specific sail or set of sails. And then overlaid with different sail options. Hard to describe with words--which is why they are graphs. The numbers can be determined by sailing and noting, or by running a velocity prediction program (VPP) to generate numbers used to plot them. For boats that are raced, someone has usually paid to get charts, or an owners association has access to them. The USYRU should have them available--for a fee. But you also may be able to find a free VPP program on the web, there used to be at least one around, if you really hunted for it.
You have to take them with a grain of salt, since there are always intangibles (sea state, offsetting current, etc.) and there's room for some interpretation, i.e. if they show two different mainsail sizes, and you've got one sail with a deep reef in it, your reef will create turbulence and never quite be the same as the smaller "real" sail would be.
But in general they can show you the fastest points of sail (for boat speed and VMG alike, upwind or down) as well as the most effective sail combinations, i.e. if they show the boat will be faster with less sail--they're usually RIGHT about it, and reefing early may pay off. While they're designed for racing purposes, they can help anyone come up to speed (no pun intended) about how to best trim a boat. If you have a local PHRF group, they may also be able to hook you up with something.