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Where N is the number of services the key unlocks, is there a O(1) migration story yet?I'm not 100% sure I'm interpreting what you meant by this correctly, but if you're saying essentially "is there a single click button that will change all my existing accounts over" the answer is a mixture of "no of course" and "there wouldn't be one answer anyway". I mean, there are still plenty of people who don't even use password managers or have any real formal list of services at all. How would you expect there to be any automated migration there? And even with a password manager it'd still be pretty complex to manage some sort of automated switchover given the diversity of credentials and back ends (remember we're not just dealing with websites), even beyond user interaction being required for each use of many hardware key systems (which is indeed good security practice).
However I don't think that matters much having already been through a service migration ages ago with the rise of password managers themselves (and being a fairly early adopter). What happened there (or for email migrations for that matter) was a basic piecemeal/spread-it-out approach. A handful of critical services got changed right away, and then over time I switched others from using old bad passwords of a small set to being listed/managed by password manager(s) whenever I accessed them. The most frequently accessed happened the quickest, others eventually, and I made some systematic effort to get some of the long tail I cared about while just abandoning a few of the very oldest.
As it becomes more universal and everyone gets comfortable with it being battle tested, kinks worked out, rich ecosystems etc, I suspect we'll then start to see sites actively prompting/requiring people to add or switch, which will then happen naturally in the course of normal usage. I currently have almost 1000 items in my password manager (though granted not all of them are passwords) which is intimidating on the face of it, but previous experience tells me it won't actually be a big deal. A few dozen already have keys registered, and over time that will naturally increase as services/systems add support. Well before this time in 2032 I expect everything to be switched that's still actively maintained. Even IRC allowed moving to using client certs instead of passwords a while ago!