Best rationalization I can think of, but I still don't approve of the change. Let us remember that Quirrell intends to help Harry become a Dark Wizard, in which case, since Harry is in the classroom, he should include Light Wizards in the class of people who can threaten the students present.

It also makes more sense to say "the adult wizard" since that sentence is the conclusion of a list of species that are dangerous, and "adult" sounds more biological.

Maybe there's an important reason for this change, but otherwise I think this is too much like a composer making inane changes to a piece after it's already written, or like George Lucas messing with the original Star Wars trilogy.