Is it possible to change my AppleCare plan?

Change how? Or to what?

If you subscribed to the one time purchase Apple Care+, you can change to the monthly version once that ends and as long as you are within one of the supported countries. click here ➜ Extend your AppleCare+ coverage - Apple Support

If you are subscribed to the monthly version, you cannot change to the 1 time payment version.

Beyond that, if you want to change from the regular Apple Care+ to Apple Care+ with theft and loss coverage, you can only do so if you are still within 60 days of having purchased the device.

click here ➜ AppleCare Products - iPhone - Apple