I was having the FMC799 plan(2Mbps till 8GB, then 512kbps) and wanted to upgrade to one of their new plans, ADBB1099 plan (2Mbps till 18GB, then 750kbps).I discovered that changing the plan was not as straightforward as calling the CC and placing the request. You should have Zero outstanding bill amount. Ideally, change the plan at billing date. You should pay advance rental equivalent to new plan rental or more to effect the change. Here is how you do it. 1. Suppose your billing date is 15th and you want to change from FMC799 to ADBB1099.2. On the 13th or 14th, pay Rs 878 (your normal bill which would be generated on 15th) + 1099 (rental of new plan you want to opt for). You can do this online through the Selfcare portal.3. On 14th evening, call the toll free CC number and tell them to change your plan to ADBB 1099, informing them that you have paid up the scheduled outstanding bill amount plus additional advance rental too.4. Your request will be placed (and you will be placed on hold for a minute or two). You will be informed that the plan change would be effected within 24hrs.5. Check Service details in Selfcare portal to confirm that the change in plan has been effected (past midnight -24hrs).Note: The amounts would be adjusted against the to-be-generated bills!