In this paper, we advocate the problem of reusing Multiplayer Online Game (MOG) in Client-Server (C/S) architecture. The problem is that MOG services cannot continue to be provided because of high maintenance cost for operating game servers. Additionally, it is caused by the decreasing service users who getting tired of the game and game providers who have faced difficulties in collecting charge of the game service. It is important that every user can play MOGs at any time whether game servers of game providers run them or not. We describe a challenging method for solving the problem. Our solution provides a middleware which is inserted under the game applications to switch network architecture from C/S to Peer-to-Peer (P2P). By exploiting this method, the network architecture of MOGs can be easily changed from C/S to P2P under some restrictions without modifying binaries. Categories and Subject Descriptors H.4 [Information Systems Applications]: Miscellaneous General Terms Management...