I have AppleCare+ on my 2019 i9 iMac. Unfortunately, AppleCare One won't work for it because it's more than four years old.
Further, once you get AppleCare One, and a device's age gets to four years, it appears you're out of luck -- Apple won't warranty it anymore, and you can't switch back to AppleCare +. On the other hand, if you instead purchased AppleCare + for that device, then you can extend the warranty on it indefinitely once the original 3-year period expires.
Another issue with AppleCare One is that it's a monthly subscription service. If you have AppleCare+ for three years on a device and you want to sell it after, say, two years, you can transfer the warranty. But once you go onto a monthly plan like AppleCare One, then warranty transfer is not possible, thus reducing the resale value of that product.
I.e., when you purchase AppleCare+, that's considered to be associated with the product. So if you sell the product, AppleCare+ transfers along with it. On the other hand, Apple considers monthly subscription services to be associated with the person who signed up for that service, so it does not transfer when you sell the product.
Granted, most people don't sell Apple devices that are less than three years old, but it's something to keep in mind.