How long does it take for a rate plan change?

Oh, I'm also having this issue;
MINs Remaining
As Of: 03/28
Anytime: 1,400
N&W: Unlimited
M2M: Unlimited
RO: 3,100
Rate Plan Overage: 50
See att.com/starservices for applicable limitations.

The worst thing you can do is change your rate plan mid cycle and u get a prorated charge for your old plan AND new plan, but also your minutes are prorated too for your new plan depending on how many days you are on the new plan until your cycle starts. In addition to that, you will see the prorated charge as well as an advanced charge for the next month. So warning; (att reps are told to adv the cust of this too) Dont make rate plan or feature changes mid cycle. Have them set to change the FIRST day of your new cycle, not a day before or a day after.

I changed my plan the first day of the new billing cycle, I called AT&T and they say that there's a glitch in the system and that it will be resolved by the end of the billing cycle, and that if I incur fee's AT&T will credit the fees.