The race was moved because if it remained the third leg of the Triple Crown, it could have conflicted with the Breeders’ Cup which in the eyes of many is the most important race day/weekend of the year. The Breeders’ Cup. is in early November and with the Ky. Derby moved to early September, the traditional gaps could have made for tough calls for connections to either race in New York in say late October or in Kentucky (Br.Cup) in early November and most people would have opted for the matter.
The length was changed because of the shift to being the first race of the Triple Crown. Some had said they were leery of sending a 3-year-old horse into a longer race this time of the year.
It is part of the prep work.
90% of the horses that ever run in the Belmont are not ready for 1 1/2 miles. 95% of them never see that distance again. As far as training them for the distance, this year is no different from previous years. Running a horse in the Derby and Preakness in the previous 6 weeks does not train a horse up to a 12 furlong race, just the opposite.