"ASAP" doesn't work that way. You have to wait for the next billing cycle to change the plan.
In short if you take up any plan you will have to stick with it for 1 month anyhow.
No sir. It doesn't work that way. It is not easy to synchronize with billing cycle even if you want to. They will switch your plan when you call them on 1504 from the same landline. The switch may take upto 24 hours.
You must wait for their phone call to confirm the switch.
During the wait period between the day of your call to them and their confirmation call, I suggest you be conservative. What I mean is that you assume the worst case plan scenario when you use your internet.
For example, when you are shifting from unlimited to NU, you curtail your day usage the day you make the call. I also suggest you save the usage details page at the time when you get their confirmation call. This comes in handy if you have to convince them in case they screw up your billing.