Think proration applies for new plan or change of plan only.Wait, that means they’ll auto deduct 100GB / 30 day = 3.33GB per day? Even if you use less than 3.33GB per day they’ll deduct it daily.
Like that if whole month don’t use data and on the last day cannot use all 100GB at once?
I recall when we first port in they also prorate to the end of the mth. That's why dangerous to port in end of the mth for lower plan because may not have enough.
Next month should reset to 100GB/100GB. Then back to normal. You can whole month don't use and use 100 GB on last day.
Those who miss change of plan on 7th should be able to verify the proration. Eg if change of plan takes place on 15 Jun means prorated 53GB, if change on 21 Jun means prorated 33GB.
Let's see when they feedback.