I have AT&T and got the same notice, and it didn't really bother me.
I am on auto pay to a credit card, which means it will just change the date it hits my card and isn't really a big deal. As stated with others, if it were coming out of a checking account and it came before a payday meant to cover it, yeah, that's not cool, but they do give you other options. And if you are really bothered, you can just manually set up a reminder to electronically pay when you want.
This isn't unusual, other companies make changes internally and move to reconcile billing and receiving. I have had dates changed with other payments as well. Just something you are aware of, and you adjust and move on. It isn't double dipping as it evens out.