Which Apple products are eligible to add to an AppleCare One plan?
Any product that was recently purchased or is currently covered by AppleCare+ can be covered in your existing AppleCare One plan. You can also add devices you already own that are in good condition (a diagnostic check and/or inspection may be required) and four years old or newer or any headphones one year old or newer.
这里面提到可能会有diagnostic check才能加，不知道会不会严格执行