The technical changes they would make to the group for 12 speed cassettes already exist. They're already in three MTB groups. They just need to patch them into DA and add a click to the shifter.
As for schedule, SRAM have been pushing them. Shimano finally brought out 11 speed MTB XTR in 2014, still with a triple and double option. It wasn't the right move to catch SRAM who brought out 12 speed Eagle in 2016 and had it through their whole lineup by 2018. So Shimano has to accelerate, and this year here's SLX 12 speed at the same time as XT, instead of a year later.
A running change would not be unprecedented. The 9/10 speed change for MTB happened mid-cycle, there are some derailleurs that are the same except for their leverage. They also introduced "29er" cassettes mid-cycle, that were 12-36 instead of 11-34
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Genesis 49:16-17
Last edited by Darth Lefty; 07-08-19 at 09:40 AM.