There are two key dates - your requalification date and your account anniversary date.

Historically there used to be a big gap between these, ie you'd requalify on the 10th Sept but your new status didn't apply until the 1st Nov.

It's my understanding now that if your requalification date is before the middle of the month your account anniversary date is the 1st of the following month (ie 1st Oct) and if the requalification date is after that is the start of the month after that (ie 1st Nov).

You can see this in your account.