Changing a 2 degree side bevel to a 3 degree - is it just a matter of setting your edge guide to 87 degree and file the edges?
If the sidewall (non metal part of the ski next to the edge) interferes with filing the edge to three degrees, then you will take a very long time to get that edge filed, and your file will get gummed up; it's better to use a sidewall planer first.
If you have lots of room, and the file doesn't touch the sidewall at 3 degrees, don't worry about it.
If you have Volant's, don't do it! (leave 'em at 1,1 or you might be able to squeeze 2 degrees in once, if your very lucky and they are new - don't ask me how I know).
What, you don't like shaving metal topsheet/cap ?
No. There is a thread in this forum that discusses this and Terry (SlideWright) did a good job of explaining this with diagrams. The actual effort to go from one edge setting to another is the same. People, myself included, tend to jump on the "You'll waste lots of material!" bandwagon. In all actuality, it isn't that big of a deal.
The real thing to ask yourself is why you might want to do that. Once they are set to 3, I can't see anything to be gained by going back to 2.