From long time I was looking to make a change in my broadband plan and finally today the january 1st was the day it happened.
There are two ways you can do this.Call 1500 which happens to be BSNL swagatham service and tell them about your plan change.They will record it and forward it to your local customer care center.Then you may be called by the customer care executive to have your signature ( I said may be because in some instances the change happened online).
The other way is to approach the nearest customer care center and fill the change request form.It should be signed by the person to whom the landline belongs.In cases when forms are not available ( remember BSNL belongs to Govt) you can also write a letter to the BSNL manager of your city in a paper and hand it over to the CC executive.
The point to remember here is that,you make these changes atleast 10 days prior to the next month.Which means if you want your new plan to be effective from february,then call 1500 or approach cc center by 21 or 22 january.And make calls to 1500 service in these 10 days gap too and keep them reminding about this.Hopefully your new plan becomes effective from next month.
Oh and by the way I changed my plan to 500C from H250.