When should you repot orchids - do they need fresh soil?

Repotting orchids is actually pretty easy. You should do it when the plant is outgrowing the pot or when the substrate gets compacted and doesn't aerate well anymore. At home I usually check that every two to three years, depending on how the orchid's doing. The best time is right after it's done blooming, because the plant handles the stress better then.

Yeah, fresh soil is important, but not regular potting soil - your roots will rot in that. You need special orchid substrate, which is usually bark, moss and stuff like that. The stuff is loose and airy, exactly how orchids like it. When you repot, get a pot that's only a little bit bigger, not much bigger, or it'll get too wet. Carefully take out the old roots, remove the dead brown roots, and then put it in the new substrate. After that don't water for a week or two, so the plant can recover.