How does BSNL adjust billing if the broadband plan is changed on the same month?

suppose i change from plan 250 to 500c. Now, bsnl accepts the plan change faster, in two three days. So, how do they adjust the billing. Suppose i requested plan change on sep 12 and the new plan provisioned on sep 14, what bill i get for sep?

They check the usage from starting of the month till the date of plan change.Suppose if ur in 250 plan and changed to 500c on 25th of the month and used 1.5GB till 25th means they will charge you 250+100(0.20paise per MB) for additional 0.5GB usage.
If free limit is 4GB/month(presumption) in 500C for you only 0.6GB is free limit from 26th of the month to end of the month,for that they charge Rs.84{(500/30days)*5days}.For excess usage they charge 0.20p/MBadditionally.

suppose i change from plan 250 to 500c. Now, bsnl accepts the plan change faster, in two three days. So, how do they adjust the billing. Suppose i requested plan change on sep 12 and the new plan provisioned on sep 14, what bill i get for sep?

Suppose

1. Current plan = Home 500 C - 1.5 GB day time usage - Till 10 th

2. New plan = Home 600 C - 2.5 GB day time usage - Active from 11 th

Your day time usage limit for first 10 days will be

1.5 x (10/30) = 0.5 GB. If your day time usage is more than that, you need to pay for it (irrespective of your new plan).

Your day time usage limit for next 20 days will be

2.5 x (20/30) = 1.67 GB

If your day time usage is more than that for this 20 days, you need to pay extra (irrespective of your previous plan)

Hope it is clear now.