Understand that if customer re-install or fresh install the app on the new phone, a new ECID will be generated and thus counted as a new UV.
But what about those cases when the app is transferred from the old phone to new phone, e.g. by using iTune backup for iPhones; using tools like Smart Switch for Samsung; is the ECID going to be unchanged and thus same UV is preserved.
Based on my understanding the Visitor Id changes when the visitor switches to a different phone. However, if you want the Visitor ID to be the same then you can try using the setUserIdentifier Mobile SDK API to set your custom visitor id as you can populate the custom visitor id for the visitor once the visitor signs in to your App. For more information, you can refer to the below documentation link.
Hope this helps.