In the same way that Letterman was allowed to finish out his run on NBC before going to CBS and wasn't terribly gentle with NBC/GE, I'm betting that as long as Colbert isn't egregious (and yeah, that's a moving target these days and may not be the same if Ellison gets the keys before the end of the run), it stays on. CBS has already collected tens of millions of dollars in advertising commitments for a season of Late Show with Stephen Colbert---new episodes, not repeats.
What was also known at that time regarding Letterman finishing out his run on NBC was that the lame duck period (Between the time that his departure from NBC for CBS had been announced to when Letterman did his final NBC show.) was practically 6 Months and the advertising commitments NBC had for the remaining Letterman hosted Late Night episodes were set until the end of June of that year (1993). And in one of the Late Night opens that had been done in the final weeks of Letterman's NBC run had a computer generated Kite that had the G.E. Sucks written on it in Japanese.
The lame duck period that Colbert has here is 10 months basically and what he needs to do is to tread real carefully.
