Going from a quad cab topper and putting that topper n a crew cab will end up with a larger gap at the top of the crew cab. the quad cab topper truck bed has a rather curved angle to the bed itself, against the truck cab boy. once it reaches the top of the bed rails of the quad cab, the topper no longer continues along that curved lien and is much more verticle. to match up with the styling of the wuad cabs rear window and roof.
However, the new crew cab will continue along that curved line and your old quad topper, on the new crew cab truck, will not match up with and be further away, at the top fo the truck cab and topper, the gap will be lareger than at the bottom down by the bed rails.