If I Book a campsite but get there early enough that I get/change my booked campsite to a primitive campsites that are only first come first serve, would that be possible?

This also depends on when. On holiday weekends most sites are reserved. But when the park is more empty they are much more laid back about letting you swap.

Just give them a call. The place is totally a awesome place with a great staff to assist you.

Not sure. You will need to call

You need to call them and ask them! The Gennie Springs facility is excellent. They have lot's of thing's to do and I think reasonable.

Just call Ginnie Springs and ask about their 1st come first serve policy