I am not sure where you got the 15th date from unless that is your bill cutoff. Here are the details about the change.
Customers are given 30-day notice then migrated to new plans where 3G and 4G usage contributes toward the same data allowance bucket.
30-day notification details for customers activated on a 3G/4G plan:
Receive the bill notification or postcard in October 2011.
Change is effective on the first day of their November bill cycle
Activated Between 9/21/11 and 10/19/11:
Receive the bill notification or postcard in November 2011.
Change is effective on the first day of their December bill cycle.
The usage can be delayed due to towers reporting. If you want up to the minute usage I recommend getting a program on your PC to keep up to the current data session. To be absolutely accurate on your data usage and to eliminate the guesswork, delays, confusion, frustrations and uncertainty you may want to try one of these. The only one I have used is duMeter and I like it.