Since it was confirmed that Daniel Ricciardo will be replacing Nyck De Vries at Alpha Tauri from the Hungarian GP onwards, I've noticed comments suggesting that De Vries would have been better accepting a deal to drive for Williams for this year (I tend to agree, as his previous ties with Mercedes would have been more useful and I would have expected him to at least see out the season). This got me thinking on a tangent about drivers who switched teams at the exact right time, and conversely those drivers who could not have timed their team changes worse.
For a driver making the move at the right time, I am primarily thinking about examples where a driver moved from what up to that point had been a more competitive team than the one he switched to, but where his new team improved and/or his old team declined after the move. For a driver switching teams at the wrong time, I am considering the opposite scenario (in other words, the driver's career would likely have been better staying with his previous team).
I will start with a few (mostly relatively recent) examples of well timed moves, and give one example of a badly timed move (on the grounds that I think this particular case takes some beating!).
Well timed driver moves
Lewis Hamilton - McLaren to Mercedes (2013). This is probably the most obvious example of the modern era, but it is obvious for a reason. While Mercedes had achieved their first GP win as a constructor since 1955 during the 2012 season, this looked a very risky move for Hamilton as the team dropped down the order as the year progressed. By contrast, McLaren had a very competitive car that won as many races as the WCC winning team (Red Bull) and, despite operational and reliability issues which meant the team could not ultimately maintain its championship challenge to the end of the season, seemed well placed to continue to win races. However, the 2013 Mercedes (although a notorious tyre eater) was better than expected, allowing Hamilton and Rosberg to win a total of three races, while McLaren's new car concept produced a dud that failed to deliver even a single podium finish and finished well behind the top four teams. I think the results for the respective teams in the seasons that follow speak for themselves and there is now absolutely no doubt that Hamilton made the correct decision.
Fernando Alonso - Renault to McLaren (2007). While several of Alonso's subsequent career moves have, I think he got this one spot on. It's worth remembering that this switch was announced at the end of 2005, when McLaren had an extremely fast (if unreliable) car. In 2006 it looked like Alonso might have made the wrong decision as he won his second consecutive title for Renault while McLaren finished the season winless and a distant third in the WCC, scoring just over half Renault's points tally. However, McLaren adapted much better to the new 'control' Bridgestone tyres for 2007 than Renault and returned to competitive form, allowing Alonso to continue winning races and fall just short of the title even with his rookie team mate proving far more of a nuisance than he'd anticipated! The Renault team fell back into the midfield, finishing with just a single podium and picking up most of their points from minor placings. Alonso would have stood no chance of defending his title in the 2007 Renault, so even allowing for his subsequent spectacular fall out with McLaren this was the correct move.
Jenson Button - Brawn to McLaren (2010). I fully expected Button to stay with the team that helped make him a world champion and was very surprised when it was announced that he would be joining Hamilton at McLaren, who had admittedly recovered fairly well from a shocking start to 2009. However, Button's switch proved correct, as he performed well overall against Hamilton (especially in 2011) and added another eight victories to his tally, burnishing his credentials as a worthy world champion, whereas Mercedes fell back towards the pack and won just one race with a handful of other podiums in the next three years. Brawn's lack of funding during the 2009 season perhaps makes this example more predictable than the ones above, but Button's brave move was vindicated.
Badly timed driver moves
Fernando Alonso - Ferrari to McLaren (2015). Where to start with this one? The 2014 Ferrari was far from impressive and I think the fact that they finished above McLaren at all had a lot to do with Alonso's impressive results, including two podiums, and this combined with his utter demolition of Kimi Raikkonen reflect better on his capabilities than the car's. McLaren didn't shine either that year truth be told, being outperformed as the leading Mercedes customer team by Williams, but their 2014 season was positively glorious compared to the unmitigated disaster that was 2015, with the underpowered and unreliable Honda engine married to a chassis that seemed to be nowhere near as good as the team claimed it was. Making matters worse for Alonso, Ferrari improved significantly and while still unable to consistently challenge the dominant Mercedes pair, finished runners up with Vettel taking three wins and a comfortable third in the WDC, while Alonso toiled with only fifth place in the Hungarian GP as respite. Even that result came in a race won by Vettel! I would argue that this is the single worst driver switch of the 21st century to date.