You can modify an ASN on a 7210 SAS but the new ASN will not be used until the BGP instance is restarted either by administratively disabling or enabling the BGP instance or by rebooting the system with the new configuration.
Since the ASN is defined in the config>router context, not in the BGP configuration context, the BGP instance is not aware of the change. Re-examine the plan detailing the autonomous system the SRs belonging to each group, group names, and peering connections. Changing an ASN on a 7210 SAS could cause configuration inconsistencies if associated peer-as values are not also modified as required. At the group and neighbor levels, BGP will re-establish the peer relationships with all peers in the group with the new ASN.
Use the following syntax to change an ASN.
config>router# autonomous-system autonomous-system
config>router# bgp
group name
neighbor ip-addr
peer-as asn
config>router# autonomous-system 400
config>router# bgp
config>router>bgp# group headquarters1
config>router>bgp>group# neighbor 10.10.10.103
config>router>bgp>group# peer-as 400
config>router>bgp>group# exit