Before upgrading from 6.3 to 6.4 please ensure that all gateway AMIs are on a supported version. To do so please go to:
Controller > Settings > Maintenance > Software Patches > Generate list of Aviatrix Gateways using deprecated AMIs > 3 dot icon > Apply Patch
Once applied a list of deprecated Gateway AMIs will be emailed to admin account's email address. The gateways listed will need to be replaced before upgrading to 6.4.
For more information on Gateway Replace please see link below: https://docs.aviatrix.com/HowTos/Troubleshoot_Diagnostics.html?#gateway-replace