Changed Plans a week into Bill - I still have to pay for the old plan for the net 3 weeks?

AT&T -

Hoping for some help -

We changed our old plan from 5 phones with individual data plans to the 15GB Shared Plan on 4/10 - My bill is due 4/28.  My Bill still have the old amount due for the month - but yet my Data Allocation changed instantly to 15GB form my old 19GB for all 5 phones.

I spoke to someone in Chat and he said thats right, I will have to pay for the old plan and the new one will kick in next bill.

That makes alittle sense, but why would AT&T switch my plan to less data and still charge me for the old plan????

Any help?!?!?!

Thanks