My not very interesting answer is not much will change for the Summer hosts. If Sydney bids again for 2008, I'd go as far as to say nothing would change at all, apart from swapping 2000 and 2008 around. So:
Sydney 2008 if they bid again, if not Toronto 2008
Rio 2016 or Tokyo 2016
Tokyo 2020 or Rio 2020
2012: perhaps Paris might have a smaller edge as an Anglophone country/city hosted the Games the year before the 2009 Copenhagen IOC session
2016: in the Sydney 2008 timeline nothing changes, in the Toronto 2008 timeline there's less of a clamour for an American Games and more of a clamour for an Asian Games so Tokyo wins, but Rio has much momentum.
2020: as in our world, the loser in the 2016 race wins. The intriguing point in the "Toronto 2008/Tokyo 2016" timeline is the IOC would be awarding Rio the Games the year before the 2014 world cup. Would world headlines about stadium prep, civil disorder, protests etc put them off or would momentum carry the day? I'm going for the latter. The IOC is confident after a run of successes (after the previous blowouts of Beijing 2000 and Athens 2004). Sydney/Toronto and London/Paris have been big successes and preps for Tokyo are going well. So they take a punt on Brazil (and the landslide with which they won 2016 in our world suggests this is likely too).