Can a first author of a pre-print be demoted in the version sent to a journal?
Yes.

Preprints are preprints. They're preliminary versions. It's not surprising if the preprint differs from the version that's submitted to a journal. Author orders can even switch during revisions, during publication, etc.

You also ask about the first author "defending" himself/herself against this, which implies you think the order switch is unethical. In that case, speak to the other authors first, argue why the first author deserves to be the first author, and if they can't be convinced (+ you're equally convinced of your position), that's when you escalate.