Modificar ASN_Direct_Connet_Gateway

Yes, it is possible with CLI, recently this new feature was added for TGW (currently not possible via AWS Console, CLI only)

Info on the API;

Also note: "The modify ASN operation is not allowed on a transit gateway with active BGP sessions. You must first delete all transit gateway attachments that have BGP configured prior to modifying the ASN on the transit gateway."

Example command:

aws ec2 modify-transit-gateway --transit-gateway-id tgw-1234567 --options AmazonSideAsn=65002

As for the DirectConnectGateway - you can not modify the BGP ASN once it is setup; you will need to delete and recreate a new DXGW with the desired BGP ASN. More information on the BGP ASN in the DX FAQ