You would need to reach out to BSNL, they should be able to change your plan. It wouldn't be possible over the phone or through the self care portal, you would need to visit a BSNL CSC or exchange.
There is long term plan for Superstar 300 as well.
Regarding how it would be adjusted, most likely they'd charge you the prorated amount and the excess would be added as account credit which would be applied to your new plan. If you go for the monthly option, I think it would remain as account credit and you wouldn't need to pay the bills until the net amount due becomes greater than zero.