Is that an undocumented feature? I don’t recall ever hearing that advice before this. Once it’s in the footprint editor, I assume you can save it as a new footprint(s) and it will be automagically saved in the current format (compatible with the “.pretty” footprint libraries).
Several years ago I recall converting some old-format libraries to the *.pretty format, using the procedure described by @keruseykaryu in the thread Question about .mod and .pretty files.
Dale