Duo recently added a feature (haven’t tested it yet) that allows you to transfer to another phone.

Quote from email:

In late August we will release an updated version of the Duo Mobile app that includes a new feature called Duo Restore. This functionality enables Android and iOS Duo Mobile users to recover their Duo-protected accounts when they get a new device.

Duo Restore is an opt-in feature that can be enabled or disabled by Duo Administrators in the Duo Admin Panel. It is not enabled by default.