The main reason to change calls is "we've always done it that way".
Of late, many format changes have not been accompanied by call changes. Nobody cares.
About the only reason to change today is to link the name to the station at the buyer level. One place where calls are totally important is at the agency level. Ratings analysis and orders are done based on the calls. If the calls match the name of the station, it's easier for buyers in other cities to recognize the station.
It seems like most commercial radio stations have gotten pretty savvy about burying their calls and legal ID deep within station imaging to the point that most people wouldn't even notice.