I have already done it to the 7 websites that I own. Anyone not upgrading is just going to stay stuck with with they have until they upgrade. I mean, I loved XF1, but even I would know that it had some areas that needed to be upgraded. I upgraded without hesitation. I ran some tests, but other than that, I was really not hesitant. I even upgraded my backend from MySQL 5.2 to 5.7. Then PHP? 5.4 all the way up to 7.2. On top of that, I grabbed a nice custom skin from Pixel freakin' Exit!