Existing t2.micro instance updated to t3.micro with Ubuntu 20.04 upgrade - no network interface

https://aws.amazon.com/premiumsupport/knowledge-center/ec2-ubuntu-secondary-network-interface/

This is not exactly what your issue is, but maybe it can point you in the right direction?

Make sure the config file is correct? /etc/netplan/51-eth1.yaml and/or maybe simply running netplan --debug apply might be what's missing?

Under the hood, the t2 and t3 instance types use different virtualisation (t2 uses Xen, t3 uses Nitro). The network drivers required by the operating system change and that's why you can't see the network interface once you move to a t3 instance type.

Before performing the change, ensure that you install the ENA driver. Although these instructions reference Red Hat you should be able to extrapolate for Ubuntu.