If you get a new smartphone or mobile device, you need to add the new phone (and remove the old one) by visiting the HarvardKey self-service portal and choosing "Set-up & Manage Your Two-Step Verification" in "Manage Your Services & Account." If the number is the same, you can get a phone call or SMS text message there in order to get into the Manage Devices page. If you still have your old phone, you could also use the app on that to authenticate with the "Push" feature.
Once into the Manage Devices page in HarvardKey, remove your old phone first. Then add the new phone (installing the Duo application as you register it).