Reverse Gravity vs Air Walk

[ckdragons]

I understand that flying creatures are unaffected by the reverse gravity spell. What about creatures using air walk? Do those creatures "fall upwards" when they are "air walking" on the ground?

[Gulthor]

Their orientation should invert, but they wouldn't fall - an air walking character can be knocked prone, but they treat the air beneath them as a solid surface. I would probably have them make an Acrobatics check to avoid falling prone as they adjust to their new orientation - probably a DC 10 or 15 or so.