FCC rules at the time the Evil News Association and Post/Newsweek swapped their respective TV stations in Detroit and Washington D.C. did not allow for broadcast stations to share call letters unless they were among co-owned facilities in the same community. Channel 4 had to adopt new call letters once the swap was complete, although not immediately. The same thing happened when Meredith Broadcasting bought WNEM-TV in Bay City, which forced WNEM-FM to become WGER-FM 102.5 and when Time-Life Broadcasting purchased WOOD-TV in Grand Rapids, which became WOTV since WOOD-AM and WOOD-FM retained those call letters. Unlike today, where a corporation can share call letters among different stations it owns in different parts of the county. KCBS-TV in Los Angeles was initially KNXT because the KCBS call sign was already used by it's FM station in San Francisco. When the FCC relaxed the call letter requirement CBS changed the Los Angeles station to KCBS-TV, which it retains today.It’s to bad Channel 4 didn’t keep the WWJ Heritage calls, Why did the get rid of them in the first place? Stupid move on whoever owned the station at the time."
(CORRECTING MYSELF HERE!) The KCBS calls were originally in SF but on AM 740 not FM. As another poster correctly added, KNXT was named after KNX-AM in LA. But again the FCC rules until the early '80s prohibited the sharing of similar call letters between stations in different geographical areas. Now they don't even have to be owned by the same company. Case in point, WJMN is shared by the CBS affiliate in Escanaba/Marquette/Iron Mountain, MI and 94.5 FM in Boston, MA. I've been told the Boston station pays a fee to WJMN's parent company for sharing the calls. But I haven't been able to confirm it. Knowing that WJMN-TV and sister station WFRV-TV Green Bay, WI are owned by DeathStar Media, it wouldn't surprise me at all.