Checksum basically adds up some of the code, and converts into a value, and compares it with the integer stored in the client. This prevents bad code/code that didn't download properly.
Changing the checksum isn't literal, but you can try changing the checksum value that it looks for, to the one your edited file is. However, I believe it will be hard coded, even in Infantry, so it won't be editable by a hex editor, you might have to reverse engineer the client, however your best bet is to disable the checksum program, which I think is in the updater in the client.