I was listening to K-104 pretty exclusively from 1978-82 (I know, I know, who'da thought?) All I recall is that they would play some crossover hits if they were semi-compatible with their format, were in strong demand, or were by an artist that the station already played. (I have a piece of a 9/1979 aircheck where morning man Warren Epps intro's "Listen to the Music" by the Doobie Bros...apparently since the later Doobies stuff was compatible ["Takin' it to the Streets," "Minute By Minute"] they thought the ENTIRE Doobie catalog was ok...but "Listen to the Music"??? That's really a COUNTRY crossover song!)
One crossover hit they played relentlessly was Queen's "Another One Bites the Dust."
But looking at it year-by-year, the rock adds on their playlist were very few.
1978 - "K-104 is Disco Soul"-exclusively popular R&B (mostly currents) and disco hits
1979 - "K-104 is Disco"-same thing, but kinder to white artists who had disco hits. Possibly a reaction to competitor KNOK, who played more disco, and usually played the longer versions and the deeper cuts. (But 27kW KNOK was never much of a factor; I believe they beat K-104 in one book one time during all the years they competed.)
1980+ - "K-104 Music Radio"-with disco falling out of favor, they went back to exclusively popular R&B (nearly all currents) and would play select crossover hits (Doobies, Ambrosia, etc) to pull in some audience from KNUS and Z-97. They also began incorporating rap (back in the tame days of the Sugar Hill Gang, Kurtis Blow, etc.)
I have a "K-104 Top Hits of 1983" list they put out at the end of that year, which listed almost all R&B hits, so apparently any rock mixture was long gone by then. Also, Scott West would throw in an occasional smooth jazz crossover song or two during his show.
So, a long-winded answer to your question...I'd say it was 1982-83 when they went all Urban...but to me, being truly "Urban" is as much a commitment to the station participating in the black community as it is to playing specific music. K-104 didn't really push that part until KJMZ came along in late 1988 and showed 'em how it was done.
Joe King jocked there in 1982 and frequents this board. Hopefully he can shed some more light on this.