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That's already the case. USPS relied heavily on being able to move cargo in the belly of passenger planes which lead to disruptions this year. You can also fairly quickly take all the seats out of most passenger planes since they're just stuck in there with L track.

You are thinking of "combi" aircraft. In North America, you see a lot of them operating in Northern Canada and Alaska.

https://en.wikipedia.org/wiki/Combi_aircraft

I saw a TV show called Ice Pilots years ago which focused on an airline in the northern parts of Canada. If memory serves, they flied planes that had a configurable cabin where they could add rows of seats as needed and a divider leaving the rest for cargo.