Since no admins moved or closed the previous thread posted on CC modification by mistake (sorry again) i recreated it in the correct forum.
Since that the only thing i find close to what i imagined is
this tool; but its unfinished and it only can change instruments in the same ROM; nothing more. The good thing this is for several Square games, and if anyone could finish the tool a lot of work patching the roms to replace music could be done in a few clicks.