Need to Downgrade m6a.large instance running as an Active Directory Domain Controller on AWS to t3a.medium.

Aside from the CPU, memory and some networking performance, the application shouldn't notice the instance change. However, if you have an external IPv4 address configured and its not an Elastic IP (EIP) it will change. If you're services use this IP in anyway I'd strongly consider allocating and assigning an EIP, updating the resources and service that use it (ie Route53, apps and scripts, etc) before the start/stop event.

I'm assuming you're doing this for cost savings. If correct, why not consider purchasing a Reserve instance and move up to a t3a.large (or higher) instance type? An m6a.large to a t3a.medium if a significant downgrade. Buying a reserve instance will give you a little more compute power as a buffer and still keep a the cost reduction. Just a thought.