Vodafone India — prepaid number activation - unfair trade practice

I have been a vodafone prepaid customer for 5 years, but due to some reasons i couldn't use the number for 1-2 months. The number got deactivated. For reactivation i visited one of the vodafone stores in hyderabad. As per the vodafone agent/support person, the number is 'inactive, state and cannot be reactivated as prepaid number since it is premium number. Also i have to reactivate it as postpaid number and use it for 4 months.
First of all, why do i have to reactivate it as postpaid number? It was prepaid a number when it got deactivated.
Second, why do i have to use for for 4 months? I am not a new user/customer. I have been using the number from past 5 years.
It is my number. How can vodafone force its customer to such rules. Why take advantage of their helplessness as they are aware that number is being used for years and customer has no other option than forcefully migrating to postpaid to retain their own number.