The 3 months part is fine. The thing is they haven't specifically mentioned which plans will be upgraded!
New Delhi, March 21, 2010: Bharti Airtel, one of Asia’s leading integrated telecom service providers, in an unprecedented move is upgrading speeds of all its existing customers (on 64, 128, 256 Kbps) to a minimum of 512 Kbps. Further, customers currently on 512 Kbps plans and 1mbps plans will have their speeds doubled to 1Mbps and 2Mbps respectively. The upgrades come at no extra cost to existing customers and will be completed by June 2010 pan India. Airtel also announced the launch of ‘Impatience Plans’ – an industry first for broadband, by offering 4 Mbps speed plans starting at a path breaking Rs. 899/ month.
It appears they have no plans to double 2mbps to 4mbps....probably because it will eat into their 4mbps Impatient Plans segment