It depends a little on how you arrange those files, but with UnixFS (the standard way to arrange them?) the files contents are arranged into a merkle DAG, so an

It depends a little on how you arrange those files, but with UnixFS (the standard way to arrange them?) the files contents are arranged into a merkle DAG, so any shared files (and indeed chunks thereof) between the two versions would remain the same and thus still be ‘seedable’ by the parties that have it. It’s similar to BitTorrent in that regard. The root of content would get a new content address, but all the identical data below it in the merkle DAG would have the same content address as before