From how Amex insurance usually works, they won’t upgrade your cabin. If you need to change your return because of a medical issue, they’ll generally rebook you in the same cabin you originally paid for, so Premium Economy in your case.
The coverage is meant to get you home under similar conditions, not improve the ticket. If Business ends up being the only available option, sometimes they’ll cover it, but that’s more of an exception than the rule and usually depends on availability and the specific claims agent.
If this is likely, I’d call Amex Travel Insurance directly before making any changes and get it confirmed in writing. Their answers can vary, and you don’t want surprises after the fact.
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