If you unlink the media you can then download the new media. It sounds like a lot of work but I've been downloading individual media for a couple of weeks now one by one where I didn't have them. But maybe you could do the download missing media all at once. I don't think there is any thing extra added as far as source, it seems just the media has changed. Of course I only have about 9 sources in this category.
In case anyone is still wondering, this is 1911 UK census.