Now that you know how to fix checksums anyhow (because it is important to understand), let me teach you a great trick that I actually just discovered while writing this up. If you take a savestate on the continue screen, you can change your saveram any way you want, load your savestate, and continue your game without worrying about the checksum. (Please note: I'm using ZSNES 0.915c, and don't know if this trick would work on other emulators or other versions of ZSNES.) La!