Twice charging is possible yes but only for channels/packs not NCF, we have also seen from screenshots shared by other members in this thread that the deducted amount is often refunded too.
Your calculations does not seem to be correct as well. You see if Rs 13 was charged earlier that means his pack price = 13 x 30 = 390 (including 155 NCF). Then if Rs 10 is deducted again after changing pack then pack price will be 10 x 30 = 300 and this price is EXCLUDING 155 NCF because it was already charged so he haven't downgraded but he upgraded his plan from 390 per month to Rs 455 per month.