That is a very good point. And it reminds me how you can do gradual releases when you only have one installation: you do it on a per-user basis, using feature

That is a very good point. And it reminds me how you can do gradual releases when you only have one installation: you do it on a per-user basis, using feature flags. If it's only the logic behind an existing interface, you can implement both the old logic and the new logic and choose between them based on a feature flag.