According to Webster's dictionary, either plural form is admissible... I'd love to give a talk on supernova cosmology some day where throughout I use "supernovae" in the nominative, and "supernovas" in the accusative.  It would be so entertaining to watch the reactions of people in the audience...

All that said, though, I'm fine with the switch to "supernovae" here, as it's by far the more commonly used form.