Fsck to the rescue?

if system crashs/looses power between the rename hitting the disk, and the new data actually being writen, then there is a very good chance that the old data is still on the disk.

there are programs what can find deleted files, and undelete them. if ext4 left some clue in the journal as to which blocks were freed during a rename, then maybe fsck or the journal replaying code, could go and find those block containing the old file, and relink them.