Can't you? If I recall correctly, Harry tried and failed before succeeding with Partial Transfiguration, while he was still thinking in the terms of atoms. Also, you could consider transfiguring air as a partial transfiguration unless it's within a closed system, since you can't think of air as a whole or defined object. So it stands to consideration that Harry might be able to do it using the model of timeless physics like he did to achieve partial transfiguration, since we have no proof that he tried again afterwards ( that I recall ). Still, this wouldn't be the best moment to try it.

Besides, could've just tried to levitate the gun or something towards the gun he could use as a shield, and run away or retrieve his invisibility cloak or used the spare turn on the time-turner... There are countless possibilities. Even if he did appear on the corridor one hour before, when only Snape was there, he could probably convince him of acting as he did towards him... Which would've opened the possibility of another Harry hidden somewhere ready to try to take down professor Quirrell, or rather a Harry who had already sent a messenger Patronus to Dumbledore so he could quietly and without disrupting the Quidditch match take down the very charming Defence professor. Actually, this could still happen if Harry somehow retrieves his time-turner, or even if Cedric is hidden in Harry's pouch and uses it ( though I'm not sure how this would turn out... )