all of our instances scheduled for maint. are indeed system-reboot event types; it is indeed a stop/start situation.

stop/start can possibly put you on another physical - but it all depends on how aws has setup the hypervisors and their instance schedulers.

this, the maint from aws, appears to be security related - but that doesn't mean that aws is not getting folks off of old hardware if they have that desire.

I think it's more complicated than that. Because Amazon is claiming that if you let them handle it, you keep your instance data. That's not a stop-start (at least what we ordinary users can do).

It's more like a system restart with a little downtime managed by them.

You can try a stop-start yourself, but it's not guaranteed to help. And a restart yourself doesn't do anything.