From what people have told me about the contents of the first few episodes, it seems like WGN is using the same package Fox sent out to the Comedy Network and other stations that have broadcast WKRP in the last decade: the late '90s package, after the show's music and voice-overs had been substantially changed. In other words, the music changes are probably the same as in this list of late '90s music changes. The original versions of WKRP are essentially no longer available to broadcasters; there were a few broadcasters in the late '90s and early '00s who burned off the old syndication versions, with most of the music, but those tapes aren't being distributed any more.
The relatively good news is that this package does include some music that was eliminated on the commercial DVD and in the first-season episodes available on Hulu (which are just the butchered DVD versions). Here is a partial list of music that has been heard, or likely will be heard, on WGN that isn't on the DVD.
"Bailey's Show": "Jailhouse Rock" by Elvis Presley and two short scenes accompanied by licensed music
"Turkeys Away": "Dogs" by Pink Floyd
"Johnny Comes Back": "Layla" by Derek and the Dominoes
"Love Returns": "Beast of Burden" by the Rolling Stones
"I Want to Keep My Baby": "Baby" by Carla Thomas
"The Contest Nobody Could Win": All the songs/band names in the contest; "For the Love of Money" by the O'Jays
"Mama's Review": The scene with Venus in the booth, which the DVD cut so they wouldn't have to pay for the music
"Tornado": "Goon Squad" by Elvis Costello
"Young Master Carlson": "Soul Man" by Sam and Dave
"Never Leave Me, Lucille": "Everybody Rock n' Roll the Place" by Eddie Money; "Heartbreak Hotel" sung by Les
"I Do...I Do... For Now": Jennifer's "Fly Me to the Moon" doorbell
This also gives a better idea of why so many of us were furious with the DVD: the late '90s syndicated version had already slashed the musical content to the bone, taking out every piece of music that could be taken out without cutting entire scenes. The DVD went farther than that, cutting out what little music was left and taking large chunks of episode with them.