I don't think is should hurt. Once primary fermentation begins, the CO2 being produced will push out the ambient air and will protect the Apfelwein. If you topped off with additional apple juice, you will dilute the final product a little. I would leave as is.
On Deck: This will be the summer of wheat beer experiments!
Primary: Pomegranate Wheat, Apricot Wheat, Wine Beer
Secondary: 2012 Washington State Merlot, 2012 Italian Carmenere, 2012 Italian Montepulciano, 2012 Italian Barolo, 2012 Central Valley Viognier
Aging in bottle: 2010 Washington State Merlot, 2010 California Cabernet Sauvignon, 2011 Washington State Cabernet Sauvignon