Existing McMyAdmin 2 Pro customers can use their existing licence key with AMP to create Minecraft servers and do not need to purchase a separate AMP licence. For this you use the 'McMyAdmin' module. The 'McMyAdmin' and 'Minecraft' modules are identical with the sole exception of what licence types they use.
McMyAdmin 2 licences only allow AMP to create 'McMyAdmin' instances for managing Minecraft servers. A full AMP licence is required to manage other server types.
"McMyAdmin 3" is the name given to AMP when it's running the McMyAdmin module with an McMyAdmin 2 key. It's not a seprate product, nor does it include anything that's not in the standard Minecraft module in AMP.
AMP is managed differently from McMyAdmin 2 and there are a few changes in how it's initially set up. The main differences are as follows:
Like McMyAdmin, AMP when running Minecraft has a Minecraft directory in its datastore. So for the most part you can simply copy over this directory once you've finished setting up a new AMP instance to replace a McMyAdmin 2 one.