Almost every convertible top looks significantly different than the steel-top version. Like Skip said, you almost always see the shape of the steel frame as the top material is stretched over it, unlike the smooth, compound-curved top of a car with a steel roof.
And remember, most convertibles (especially '50s-'60s American cars) have a narrower rear seat than the same car in hardtop version because the folding top has to fit into those wells between the rear seat and the outer sheetmetal when the top is folded.
There is really no easy way to turn a hardtop model into a "convertible" if you are looking for accuracy. It can be done, but you'll need to do a fair mount of scratchbuilding if you want it to look realistic.
1967 Impala convertible vs. coupe. Big difference...
As you can see, not only is the convertible top not smooth like the steel top, but the shape of the sides is completely different than the steel-top car. Not every car will have this much difference between convertible and coupe, but there will almost always be visible differences.