I know I'm not saying anything that hasn't been said above lots of times, but go for it.
From a quick google, DX11 cards were first announced towards the end of 2009. So at launch in Dec 2014, DX10 cards were already a few years old. Yeah, I know DX10 cards would've been on sale for quite a while afterwards, but anyone buying after say 2011 had reasonable notice they were buying into old tech.
As for 64 bit windows, well that's been the sensible option for any new build for a looooong time now.
The only issue I could see is if this meant that people had bought the game with specs that met the requirements, but would be shut out of playing it at all from this change. I guess even providing a dev freezed version of say 2.2 or 2.3 might cause a support headache if those players were still interacting with the same universe. From what's been said, that's presumably a pretty small number of players - Really not sure myself whether a refund would be required to be fair, though it may well be the sensible course of action legally and PR wise (get me, armchair business adviser

).
But to shout again - go for it!