Hi bro, applecare+ helps us to claim incase of manufacturer defect or any accidental damages we do as you know. if battery health reached 80% within warranty period of 1 or 2 years, then it can be claimed. otherwise apple wont consider it as defect. when we have the usage and battery health reduces gradually, we can change battery alone after 2.5 to 3 years time limit which will be out of applecare+ warranty by that time.
If there is any damage we have done, and within 2 years period if we want to get the iPhone changed, then applecare+ will help to do that by paying claim fee alone.
So right now if you have 90% battery health then its not possible to change battery. your iPhone battery will still last for another 12 months atleast to reach that 80% level. You can continue using it if there is no issues.
Applecare+ claim in this case can be possible for entire iPhone and not just the battery part.