As a general rule, it's probably true that a majority of TV stations that signed on in the medium's early years (say, up to the mid-1960s) have now changed calls at least once. In many, many cases it was a joint radio-TV operation that split, with the TV station getting new calls. In others, it was a station that might have started out small and struggling, trying to change its image as times got better. But in any case, I suspect it's rarer now to find a station of that vintage that hasn't changed calls than one that has. (In New York state, for instance, the only big-three affiliates from those early years that still use their original calls are WRGB in Schenectady, WKTV in Utica and WKBW-TV in Buffalo, and of those only WKTV and WKBW are with their original networks; in all of New England, it's just WJAR Providence, WBZ-TV Boston, WMUR Manchester, WCSH Portland, WMTW Poland Spring and WABI-TV Bangor, and only WJAR, WCSH, WMTW and WABI are still with their original nets.)
The rarer and more interesting cases, I think, are the stations that have changed calls more than once while staying with the same network - the channel 30s in Connecticut (WKNB/WNBC/WHNB/WVIT), the channel 9s in Syracuse (WNYS/WIXT/WSYR), the channel 34s in Binghamton (WBJA/WMGC/WIVT).
Anything out there that can top four callsigns and one network in New Britain?