How to convert my VPC from single AZ setting to Multiple AZs

• In order to add an AZ(s) to a VPC, you would create new subnet(s) from existing space in your VPC's CIDR block, choosing the desired AZ in the process
• If your VPC has run out of address space, you would need to either delete some subnets, or add secondary CIDR block to your VPC. We have a guide on adding a CIDR block here: https://aws.amazon.com/premiumsupport/knowledge-center/vpc-modify-cidr-more-hosts/
• This process does not involve any downtime.
• As for your existing workload running in single AZ you have some options listed in the below article

https://aws.amazon.com/premiumsupport/knowledge-center/move-ec2-instance/