Description
Some applications both (a) want to allow users to edit metadata fields and (b) want to keep old values somewhere. But there is not a clear place to store old metadata in IPTC or XMP
One possible avenue is to define a markup inside a free-text field like instructions. The pro of this kind of approach is that it makes it more likely that the content will be preserved by (non-FHMWG-compliant) applications that only preserve a select subset of metadata fields. The cons are that (a) no current field in common use has a description that obviously includes this kind of work, and that (b) we'd need to define how to embed this in the text.
Another possibility is to define an XMP element where old copies of other metadata can be moved with a timestamp. This will be simpler to define, but will also mean adding at least two new tags that no tool not designed with FHMWG-awareness will expose it.