Bump.
I just went looking for this again... tried [on fileA, in PlaylistX] 'Send to' -> Folder (Replace) -> Replace The Selected File
in hope it might be doing the above....
No luck, turns out the 'Replace The Selected File' functionality there is 'copy fileB to location of fileA, copy the previous filesA's filename plus metadata to fileB, delete fileA AND removing the fileB from its' previous location...'.
Bit too destructive a convenience function for my liking and I still wish I could disable all data move/overwrite functions globally. So I thought I'd leave this here as a warning for others looking for similar things. As if you try the above in hope it might allow you to replace a playlist file with another.... this instead will require you to get back to backups & restore files from there as fileB is altered & the edit is unreversible.
c.