just changing the images would be rather easy to do, as long as you keep the naming canon (each replaced image
must have the exact name and path of the old one), the script references will match it. if you mess up there, Ren'Py will tell you a file not found, with path reference.
a bit more writework would have to be done if it comes to changing the text, you'll have to find and replace the lines inside them scripts, if you mess up there, you get error messages telling you the
first line that's not getting the expected result, so you better be right at each line from the beginning.. because it's going down to "fix one line, restart, next error, fix one line, restart, next error" you know the rest.
for how Ren'py can be stretched to it's limit, i'm not sure anyone knows the down-dead limit of that. some example how far you can stretch it just for a game, lookie here:
but yea, some several people in this thread mentioned the FET code is rather very spaghetti.