We just replaced the bath vanity and mirror. Both are much taller than what we took out. The light is now too low for the mirror. How hard is it to move the light fixture up so
it is now hanging over the mirror?
That depends. How and where is the light mounted now?
If the light is mounted to a flush j-box, you'll need to find out if there is sufficient slack in the cable(s) to move the box to the new location. If not, you may splice cable to extend to the new location only if the current j-box remains accessible, otherwise, you are looking at running new cable(s) to the new j-box location from the source(s).