Back in September they promised this[1]:
> If such change to Individual plans is to occur, we plan to grandfather-in all early adopters (meaning all current and future paid customers, up until this change) allowing them to keep their existing subscription price as long as they don’t cancel it.
My guess is that they will focus on the "subscription _price_" wording. Technically the price didn't change, since you can still pay them $10. They "just" changed the terms.
[1] https://blog.kagi.com/status-update-first-three-months#futur...