As far as I know, you have to proactively call them and find out. They don't let you know.
Update: Request No. again closed without any action. 1 tweet and FB post to airtel presence last night and I am migrated to new plan 1200 2 MBPS 60 GB within few hours in the morning today. But will wait patiently for my area to be feasible for 8 MBPS plan so that i can shift to transonic plan.
Conclusion: with my experiences above, never bother to call the customer care to raise new requests/register complaints. Tweet them, FB wall post them your concerns and your issue will be solved promptly.
PS: BTW i have a question for you guys:
If Airtel confirms that your area is not feasible for 8 MBPS plan how to check in future whether the area got feasible?? Do i have to apply again for change of plan and wait for confirmation on area feasibility update? Is there an alternative option?