Hi,
According to me for the old Assets you would be knowing the NBV of the asset as well as the Life that is changed in companies act for old asset, then you just meed to go to T-Code AS02 and in the Useful Life field enter the new Useful life of Asset as decided by Companies Act 2013, and then do Recalculate of Asset through T-Code AFAR. By this system will automatically divide the Remaining NBV with the Remaining useful life.
Regards,
Malhar.