that happened with me also. I took 128kbps unlimited plan initially and tried to change the plan to 256kbps unlimited, but they said it is not possible. I needed to wait for total 6 months before my 128kbps plan expired.
I was also having a similar doubt as per info provided in the web site. I have a unlimited plan valid upto mar 08, but I wanted to change to a vol based plan. This is the reply I got from customer care. But I have not changed my plan till now.
"Thank you for bringing your concern to our notice.
We would like to inform you that you can renew your account with the plan of your interest but however only balance amount will be carried forwarded but not the validity of your account. The validity will be based on your new plan. If you require any further assistance please revert back to us to do the needful.
Assuring you of our best services all the time
For any further assistance, please feel free to contact Customer Service at
Tata Indicom Broadband (the link contains the location wise contact numbers ) or mail us on
[email protected]. We will be delighted to assist you."