It's been going on at Delta at least since the time of the merger, and a friend of mine also received a similar call from Northwest about five years ago; this isn't new. In 2010, I received a call ~3 days prior to departure for a nonstop flight that had been downgraded from a domestic 767 to a 757; they offered me $400 to switch routings, and I accepted.

I also got a call last year before departing on a funky routing to Germany, as they downgauged a MD88 to a A319 (obviously a much less severe loss of seats. a "major downsize" as OHDL1 mentioned above). They didn't offer anything, so I declined the offer. The phone rep said that I was targeted due to an "inefficient routing," and mentioned that they normally target these passengers to switch itineraries as they tend to take more direct routings in lieu of compensation; it seemed that if the decrease in capacity was small enough, they could usually bait people away from the oversold segment by offering better routings. The flight went out without any DBs at the gate, so something worked for them.