I trade in my old phone. 😢i did this to my xs max last night and it worked just fine and as if it was a standard upgrade only issue i had was mail not syncing on the old phone but that's working fine on the new phone. everything else still worked just fine including photo library.
try "upgrading" to iOS 13 gm on the old phone using the option+click check for updates and selecting the file for your phone. you will have to download the file first but there are a few sites that have it.