We operated DC9-33RC for about 20 years. would swap between all pax and all cargo very frequently.
As a pax you would only have known if you were aware of a very slight (less than 3mm) movement of the seat rown relative to the cabin wall.
Later, when the fleet was refurbished, the open overhead hat-rack was a giveaway.
I recall no events where the cabin suffered any aesthetic deterioration as a result of the cargo ops.