There's a certain limit to the number of stations one entity can own in a particular market. It's generally anti-trust law type of stuff. Entercom didn't just buy Boston's stations; they bought all of CBS radio. It just so happens that Boston had more overlap than other markets, so they need to dump a lot of them. They're choosing to dump many of the CBS radio stations they bought rather than their existing stations.
Sort of like years ago in Boston when Fleet bought Bank Boston (before they themselves were bought by Bank of America). The merger as it was caused anti-trust issues, so they needed to sell of many of the branches as a condition of the merger. That's how Sovereign Bank (now Santander) got its foothold here, as they were the buyers.
What's ultimately going to happen to each of those stations will depend on what the eventual buyer wants to do. In the specific case of 98.5, the ratings release in the next few weeks will be a key determiner -- there's no way the buyer would want to change the format of the #1 rated sports station (unlikely WEEI passed them last quarter) but they could make adjustments to individual shows that aren't performing well.