Yeah. The real question is if a spell change in an upgrade is legitimate. There might not be a specific rule barring it, but it's also been convention that each card always upgrades to the same type.
That being said, I'm not sure it's possible for a "temporary" or "passive" overwrite, since it may end up overwriting other passives, which would make it less temporary. There's no problem with the concept past that, but it could much more effectively be done in a spell that says "Gain 1 air for each creature". This method of doing it seems a bit unnecessarily overcomplicated, and that seems to match the sundial mechanism more effectively in a permanent style.
Points for the way of meeting requirements, but after doing that, I can't really see any benefit to the actual cards from it.