This came up in my game vs Mike and in the end we just had to roll for it as the rules weren't clear. I've asked on Jake Thornton's FAQ blog post but no official response and any drive by commenters seem to be just as unsure as we were.
Sprint says you can't deliberately change levels. But glide says 'The model may drop any number of levels in a single Move or Sprint action without harm. This may be done as a deliberate movement or as the result of being blasted out of position.' And why would it mention sprint if you couldn't use it on a sprint action due to the same level restriction?
What I tried to do (and the dice said no) was make the first part of my sprint into open air, glide down, and then finish my sprint at ground level.
Given some time to muse, I might suggest as a ruling that you can sprint into open air as your second sprint move (you didn't change levels as part of your sprint, but gravity catches up as soon as your sprint is over) but not split it as I tried.
Or do you think I was just being cheesy and no level changes means no level changes full stop?
I can see this coming up most times Plague get the vestigal wings mutation so would be good to get a group ruling for some consistency!