Not sure that it would be undermining contracts retroactively.
IANAL, but the approach I'd consider is fraud/breach of contract. The previous victims settled and paid a royalty to use a patentable technology. It turns out that it was not what was represented. Ergo, refund. How it plays out surely depends a lot on regional courts, precedents, etc.
I wonder how air-tight you can make that reasoning. I suspect you could structure the settlement as paying to use the underlying technology _including_ the patents etc., rather than licensing the patent per se (so the fees paid are not directly predicated on the existence of a patent).
Be interesting to see how the courts would react to that though, and if they see it as a sign you are negotiating in bad faith.
IANAL, but the approach I'd consider is fraud/breach of contract. The previous victims settled and paid a royalty to use a patentable technology. It turns out that it was not what was represented. Ergo, refund. How it plays out surely depends a lot on regional courts, precedents, etc.