Confirmed that this works with Kontakt 5. If you toss your K5 plugin, loading a Logic patch that instantiated K5 will now load the same patch with K7 instantiated instead. Loading doesn't seem to be any slower, so I'm not sure it's necessary to re-save older patches at all.
I'm really glad I asked about this. I might have otherwise spent tens of hours rebuilding my patch collection, and worried about having to do the same in the future. Thanks again wonshu!