I applied to Penn State main campus for veterinary and biomedical science, but I was wondering if I would be able to change my major to undecided if my application is still under review.
Why don’t you ask them? Sometimes different programs have different minimum requirements so you should just ask them.
You’d basically be reapplying for DUS since you haven’t gotten a decision yet. Ask if that will take longer.
Your chances for either are slim because you applied after the priority deadline.