If you have a look inside the .sfl file with a hex editor you will see that it does indeed contain the filename of the .mpg (or whatever) it refers to. So if you change the name of the .mpg it will no longer be able to find it (even if you change the name of the .sfl).

You may be able to hack the filename in the .sfl using a hex editor. You would probably be OK if you just changed the name to one with the exact same number of characters, but if you change the length of the filename you will probably have to adjust the various chunk lengths accordingly, and since the file format isn't documented it might be tricky to work out exactly what you need to do. I think it's probably doable though if you understand the general principles of the RIFF file structure.

Mark