Clusters change call signs internally all the time. It's not an especially big paperwork issue. I'd bet the TV repack going on upstairs is generating 20 times the paperwork that a callsign change for 98.5 would create.
Where the confusion would probably be greatest - and the reason the calls probably haven't changed, and won't - is with ratings and ad agencies. Buyers still associate stations with call letters, and for them (if not for the average listener), "WSB-FM" is still tightly linked to the AC format on 98.5. You don't want to disrupt that just to make one hourly ID sound a smidge cleaner. It's the same reason CBS (and now Entercom) never reshuffled in Chicago, where there's an identical situation with WBBM(AM)'s all-news format now simulcast on FM, where there's already a rhythmic top-40 station called WBBM-FM. The WBBM(AM) simulcast still has the WCFS calls from when it was "Fresh," and there's never been a compelling reason to change that.