To answer your question though winter blend does not get as good gas mileage as a summer blend gas. And also in cold weather because your car takes longer to warm up than in winter it accounts for more gas use.
I change mine myself. I have no set minimum mile change. I prefer to change when the weather is decent and not cold out. I`m usually at around the 5-6k mile mark when I do the change. I know that is sooner than needed but I do it anyway.