Ch. 28:

Harry's knuckles had gone white on his wand by the time he stopped trying to Transfigure the air in front of his wand into a paperclip. It wouldn't have been safe to Transfigure the paperclip into gas, of course, but Harry didn't see any reason why it would be unsafe the other way around. It just wasn't supposed to be possible. But why not? Air was as real a substance as anything else...

Well, maybe that limitation did make sense. Air was disorganized, all the molecules constantly changing their relation to each other. Maybe you couldn't impose a new form on substance unless the substance was staying still long enough for you to master it, even though the atoms in solids were also constantly vibrating all the time...

This isn't conclusive, though. That failed attempt is before he sorts out partial transfiguration. However:

Harry may only use capabilities the story has already mentioned; he cannot develop wordless wandless Legilimency in the next 60 seconds. Of course, Harry may find more clever ways to use abilities he has already been established to have.

It would need to be fairly clear, I think, that Harry was re-purposing an old technique and not doing something new.