If I use ArcGIS Pro with the Named User license type, but later go off maintenance and my Named User account expires, can I switch my Named User license to Concurrent Use or Single Use?

Yes, if you have ArcGIS Desktop, you will be able to convert the license type to Concurrent Use or Single Use through My Esri, and continue to use the software and access projects. However, you will no longer receive software updates. You can choose to get back on maintenance at any time at the Esri Maintenance Program page.