> Also, is is possible to add v2 to existing torrents "retroactively"? Who would do that?

You would need the file data to do that, so torrent indexing sites could not do it. And since you need the full data you could only do it after downloading at which point it doesn't add that much value.

> Or would this solely be for new torrents moving forwards?

Indeed. v2 offers a few nice improvements but not world-changing ones. So there's no pressure to upgrade existing torrents. They'll keep working as-is.