Apparently when factory applied, the cap's paint hasn't had time to properly cure, so the film and the paint/clearcoat essentially bond. By the same token, but conversly, applying it to uncured paint can mean the paint never cures properly at all, the film can unbond, and it and the paint will deteriorate more quickly. In theory, if your film was applied quite some time after the paint, there is a better chance of removing it unscathed with a carefully used heat gun.
When I asked Beaver Coach Sales in Bend about the film's slightly peeling edges here and there on our relatively new coach a couple years back, their paint guru, Bruce, used a razor knife with great finesse to simply cut away the peeled sections. As those who have been fortunate enough to work with him know, Bruce is extremely particular about detail, and has his technique down pat, such that he cut through the film without nary a hint of a mark in the underlying paint. If anyone can give you a straight-up honest opinion about front-cap films, it is Bruce; you may or may not like what you hear, but the truth is what it is. He'll let you know whether it can be sucessfully removed, and which is best - new film or new paint every 5 years.
-Joel