Vimeo lets you do it. Youtube does not let you do it. Well now Songbox lets you do it too.
Imagine you have shared a Songbox with some really important people and you realise you've included the wrong mix of the track (or whatever). You can now go to that track in the "Tracks" page, navigate into the "Manage" section and swap out the audio file for the correct one. This will update that track everywhere it currently exists. Good eh?