Currently, I'm working as AWS devops at startup companies for years, so I think I have enough experience to consult you the best solution for your situation, not just server, service operationning but about the way it match to your team, your process.
Build an infrastructure for a system is hard, but make it fit to your team, your situation is the most important. What I aimed is your successful, it's both success for us.
With your brief, I couldn't give you detail plan. Just some brief:
* Volumes, snapshots, we could clean them depends on usage, backup. or optimize type, size.
* Server software update is essential, depends on os net work config that I could give you not the latest, but the best choice.
* Beside instances, I guess your system need to revise security group, network infrastructure, config ... for this move.
Feel free to contact me at skype: vnthienan
Best regards