As someone in the same boat (in the same Humvee, maybe?), I wore my glasses the week leading up to Beast to get used to the lack of peripheral vision. Pick up one of those bands that locks your glasses onto your head, the issued ones aren't fitted, and you'll need the extra help to get you through.
For transitioning back to contacts, no worries, you'll be fine. The first couple hours are different, but I got used to it very quickly. Notably, however, I've switched back to wearing glasses.
Final note - PLEASE, for the LOVE OF GOD - do NOT bring contacts to Beast. DO NOT. I don't care if you don't even wear them during Beast, do not even bring them to Beast lest you be tempted to wear them one day. The gas chamber (which isn't bad, you just have some trouble breathing, you feel like you have a slight sunburn, and you start producing mucus out the wazoo,) WILL BLIND YOU if you have contacts in. Case in point - a guy this past summer lost visibility in one eye because he put his contact lens in as soon as he got out of the chamber. It melted to his eye. Do not be that person - you will not commission, you will not pass Go, you will not collect $200. Please, don't be that guy.