Here is the procedure for updating plan if they tell such plan is not available:
Visit BSNL Selfcare portal (selfcare.xxx.bsnl.co.in). In the service request section, select plan change option and manually write the plan name. Give detailed description along with plan link from BSNL website. Then tweet @BSNLCorporate and to your state BSNL handle along with the plan change reference number. They will definitely change plan. No need to visit exchange and all.