When you migrate from setup where you have been using VPG ( Virtual Private Gateway ) to TG ( Transit Gateway ) it might be desirable not to reconfigure the VPN connection.
If you read through documentation it’s possible with just one caveat – tunnel will flip down/up during this process.
First find the details from your current VPN setup:
❯ aws ec2 describe-vpn-connections | jq '.VpnConnections[] | {VpnConnectionId, VpnGatewayId , TransitGatewayId}'
The above should provide you with information about your VpnID and VpgID
{
"VpnConnectionId": "vpn-1234567890",
"VpnGatewayId": "vgw-123456",
"TransitGatewayId": null
}Having this info is now sufficient to execute command which will move your VPN connection to transit gateway ( you need to have that ID at hand )
aws ec2 modify-vpn-connection --vpn-connection-id vpn-1234567890 -transit-gateway-id tgw-1234567890f
Detailed information you can find in the article here https://aws.amazon.com/premiumsupport/knowledge-center/transit-gateway-migrate-vpn/