The Horde is more naturey and wild, and the Alliance is more institutional and industrialized. I think thematically, it wouldn't be a stretch to say they would fit.
And if they did switch, the Night Elves could roam freely across Kalimdor again, and it would end the faction war since both factions would be more or less continent-locked.
But that's not what I'm getting at.
Hypothetical: Suppose for whatever reason they switch factions. If it made sense lore-wise, would you be okay with it?