As I recall, the official answer was:
Once a neutral is attacked it’s no longer neutral (not even pro axis or pro allies). It becomes full on “friendly” to the opposing side, in so far as it’s even a landing space for them similar to Dutch rules.
Thus, you can fly over during noncombat after you committed to at least one round of attack in the combat phase, which I’m reasonably confident was the final answer (but I’ve been known to be wrong).
High price to pay for switch all other neutrals, especially since I can’t think of many territories where there’s not an equivalent flight pattern re route that would work in the same number of spaces? I guess you’re thinking of attacking Spain to overfly and reinforce The Rock during noncom?