Just saw this on Reddit: https://www.reddit.com/r/ATT/comments/5x81jr/psa_new_unlimited_plans_are_not_prorated/
So I've been in the Directv previous unlimited data plan and just switched last night to it, so am I going to get double billed? I have 12 days left in this billing cycle...I strongly advise that you do not change your plan mid cycle or near end of the cycle. If you can backdate the change to the beginning of your cycle, or future date it to next cycle. If you change mid cycle you will be charged the portion of the month of the old plan, plus the full month plan of the new unlimited. The internal article states new plan is not prorated, its one tiny sentence in a huge fricken article to read. SO BE CAREFUL otherwise you may get a messed up high bill!