Autopilot uses the DJI SDK, which discontinued support for the Phantom 2 Vision and Vision+ in the most recent release. Furthermore, we are required to upgrade to the latest DJI SDK in order to add support for latest aircraft types, which include the Inspire 1 Pro, P3 Standard, P3 4K, etc.
If you want to continue using Autopilot 3.0 with the Vision and/or Vision+, you should not upgrade to Autopilot 3.1. Furthermore, we are not aware of any way to downgrade from 3.1 once installed. If you have automatic app updates enabled in iOS, you may want to consider disabling that feature.