the player sprites are changable, but there are no good tools for that. the only one that could work (tile molester) doesn't deal well with the width settings (only allows for multiples of 8, while sotn's hero sprites also use multiples of 4 and 2), thus making over half of them inaccessible.
monster sprites are compressed and someone would have to figure out how to decompress them, then recompress to fit them into the file without changing its size. one guy, gemini did, but he gave up on that and instead of at least sharing the knowledge he hoards it all to himself.