I managed to open the casing without breaking it *but I did break my pliers in the process. lol
You certainly need to knife thoroughly along the seam for it to open; after which you need to pry slowly with a pair of pliers by applying a considerable amount of force. Prudence is necessary when applying force because brute force will likely damage the casing; that means if you think adding more force will break the casing, it probably will. Knife it some more and then try again. Patience is *key*.