I don't know if I understood your problem very well. But have you ever tried to read the string file and transform it into a lower without using a library?

Like this

open("file.txt", "w").write(content) if (content := open("file.txt").read().lower()) else None

And if you have more files you can just add for loop

Like this

[ open(file, "w").write(content) if (content := open(file).read().lower()) else None for file in ["file1.txt", "file2.txt"] ]