I've made the switch on my Gen 2 FZ1 pretty recently. My experience was that the swap out was super simple, it was all plug and play. The LED beam was way too high though and dazzling oncoming traffic so i've had to adjust the reflectors to deal with that but now the beam is pretty good and not dazzling. The beam pattern for the full beams is a little odd though but doesn't cause too much of an issue as it's not often needed.
As I understand it, the law states that LED lights can't be retro-fitted to vehicles that weren't originally designed for LED lighting but I suspect that my MOT tester won't really notice. However, if he does pick it up then it's simply a case of swapping back to the halogen bulbs and off we go. In fact, the swap is so quick ( minutes) that I may as well pre-empt and swap back before I test it.
The reason why the beam pattern differs is that a halogen bulb is omni-directional and casts light in all directions and the reflector is designed to reflect accordingly. The LED bulbs usually cast light in either 2 or 3 directions which can cause beam pattern irregularities.
As I said though, on my Gen 2 FZ1 it works fine, the light is very bright and has improved the vision at night massively along with the fact that it's refreshed the look of the bike and rolled back a few years.