No Cup champion has changed teams at end of year:

Looking back at the history of the Chase, we ‘ve had just five drivers qualify knowing they ‘d be with a different organization – or not even racing – the following season. None of them were in serious contention for the title, and just one finished inside the top 5:
Year, Driver, Pending Silly Season Move, Points Finish
2005: Rusty Wallace (from Penske to retirement) – 8th
2005: Kurt Busch (from Roush to Penske) – 10th
2006: Mark Martin (from Roush to part-time schedule with Bobby Ginn) – 9th
2007: Kyle Busch (from Hendrick to Gibbs) – 5th
2008: Tony Stewart (from Gibbs to self-owned team) – 9th
[2012: Matt Kenseth (Roush to Gibbs?) – ?]
(Frontstretch)(6-29-2012)