I would suggest force the airline to restore it to the config it was in when leased, however that would require that layout to remain in the seat config section so the leasee would know what they have to set it back to when done with the lease.
The only thing I would prohibit is a leasee converting a passenger airplane into full on cargo. That is usually a one time deal and you are not going back.
But, for instance in our example, we took a "line airplane" and put in a sports charter seating layout. I would think something like that would be acceptable as long as it returns in the config it went out in along with being serviced to the lease specs. It cost us $1.315 million to make that change and will probably cost about the same to put it back. There is no way that cost should be forced on the airline that leases out the aircraft.
Classic example of special lease conversion: Iron Maiden when they flew the 757's. I think their Ed Force One 747 was left as is on the interior.
https://simpleflying.com/iron-maiden-747-now/
actually reading the above link, the 747 was not left "as is". They put in nice lay flat seats for those long flights.
