I recently secured a spot for an MOS that I'm unsure off. can I change MOS if I haven't signed my contract yet?

If you're unsure, why did you pick it? But, if it's rank you want to make and do all kinds of Army Hooah stuff, Infantry is the way to start

If something is available. Since it is likely at that point in the process where you need to sign soon, you may be very limited on options.

Yes, absolutely. You're not bound or required to do ANYTHING. If you want to change the MOS, change it. If the MOS you want isn't available then wait for it.