Your approach won't necessarily solve your problem, and the problem doesn't only exist for journaling filesystems. (The old DOS undelete tool demonstrates that.)
Modern filesystems may move files around at any time, transparently to programs that are currently running, so juggling file content around doesn't guarantee that you won't be leaving an old copy of the file in a section of the disk that's marked unused.
You'd be better off using encryption directly rather than trusting that you can fool the OS into overwriting the old data.