I think leave Charge cards as they are, the proposed scheme seems nice but it won't encourage people to take up Skirmisher cards in their decks for one simple reason: Skirmisher cards are not very useful cards in general, and there are people who will prefer to build high walls (useful in every case) instead of taking up Skirmishers to discard Charge cards (useful in only the case that the opponent has Charge).
In my oppinion, we should add the Skirmisher keyword to a few more cards, such as Veteran Militia.