You can only have one AS on a router at a time. You'll need to remove the BGP configuration and re-add it. "No out" the old BGP configuration and then you can implement the new one. Something like this should work:
no router bgp 65001
router bgp 65000
bgp router-id 192.168.1.1
network 192.168.1.0 mask 255.255.255.0
Obviously, don't do this remotely over the BGP network or you'll lock yourself out.
Your answer is exactly to the point and covers all bases. Thank you.