This seems like a good idea.
There are other idioms available. Microsoft office programs display “[Compatibility Mode]” in the title bar when dealing with older versions of the file format, but generally load and save transparently without changing the format, only bringing up a warning message when something is included in the document that wouldn’t work in the current format (and only then offering to convert).
Adobe Creative Suite is much more oppressive, automatically updgraidng an old file format to a new one on opening and then automatically saving in a non-backwards-compatible format the first time you hit Ctrl-S. I suspect it’s a commercial decision to encourage everyone to update to the newest version of the software or be unable to open documents from others who have already updated.
I might be misremembering, but I think years ago Sibelius announced that they were making their file format FORWARDS-compatible, which was extraordinary — essentially leaving gaps in the file format to be filled by future features, and in theory making it possible for any version of the software to open even much newer documents. If I remember correctly, that aspiration survived exactly 1 version and was then abandoned… understandably!