You're fine. I ferment 3 gal test batches in a 5 gallon carboy. The co2 produced by fermentation will fill the headspace and push any air out of the airlock.
Rack as you normally do and everything should be fine.
Tanglewood Brewing Co.
primary: Off the Wagon IPA,
secondary:5 Turns Vienna Lager
Kegged: Barleywine, Lawn Chair Pilz