Can passenger versions get converted to cargo?

edit: hmm, looked into it for a minute, and while it has been done, it doesn't sound very popular at the moment due to cost: http://www.aircargopedia.com/passengertofreightpg.htm https://centreforaviation.com/analysis/reports/air-cargo-boo...

That was the sales pitch when most observers thought the 747 was going to be the last subsonic long-haul passenger plane. "Don't worry about the supersonic plane we're currently designing, you'll be able to sell your 747 as a great easy-conversion cargo plane once the SST comes out."

Certainly can, even temporarily (this is an A380, but I assume it works for any aircraft).

https://www.cnn.com/travel/article/a380-hi-fly-auxilary-frei...