As informed by Dev, it is strongly advised not to play with the MM Posting Periods. The best way to activate batch management for a material is to clear the stock and open/pending documents (Orders, Reservations etc.) and then change the material code via tcode MM02 by ticking the batch management checkbox in Purchasing view.
Still if there is some apprehension or difficulty following this process, then another comfortable option is to create a new material exactly as the old material, tick the batch management check box in the new material, transfer the stock of the old material to the new material (If any stock exists), and set the deletion flag for the old material via tcode MM06.