So to make sure you are giving a constant current (brightness) the driver will change the voltage to give what the LED wants. That's why a III on a "V" setting works, or a III on the K2, etc...
ONLY on downconversion of voltage. Not upconversion. That requires a circuit called a charge pump and from what I know they are not built into a buckpuck, a CF board, or Ultra's board.

Don't try to simplify this for people. You will get their LED's fried.

If you take a Crystal Focus board, boot it up and power on with a luxeon V. Then hit the ignition switch again (powering down the blade, but the board is still alive) replace with a red luxeon III, and power on... the LED will fry.

Don't do it to prove me wrong, you will lose your LED.

Why?

Because when Erv' made the CF board, he did build it to regulate current YES. But.... the way he did it is on first power on it detects and sets the required voltage for THAT LED to hit the requested current value in the file. If you "hot swap" the LED's and re-ignite the blade, then your CF board will go straight back to the voltage it just detected before to hit that 1 amp value... 6.85v on a Lux III. Which fries it. The work around is you MUST reset Erv's board when you swap an LED.

A buckpuck will work for hot swap. But it works differently. Just like you have assumed, it drives entirely based on current and it detects each time. Not on the first boot like a CF board.

Now for the ultra board? I really don't know what it would do. It depends on how it's built. I will ASSUME that since it has a "setting" for luxeon V that it will go for an average current for an LED with that high forward voltage. But it could still go either way based on how Alex designed it.

So short story... don't simplify it too much for people. You cannot leave some parts out or some LED's will get fried. You can't leave out a low voltage input with a high end board will result in a dimmer LED. You'll then get all kinds of questions asking why that's going on.