Backend setup > Storage Directories. Select 'Videos' and delete '/var/lib/mythtv/videos'. The entry will default to '/'.
When exiting the setup, it will complain that '//.test/' cannot be written, select 'No thanks, I know what I'm doing'.
In the frontend, Utilitites/Setup > Setup > Media Settings > Media Settings > Video Settings > General, check 'Directories to hold videos' is showing the correct path to the nfs share.
In 'Watch Videos', press 'M' and scan for changes. All videos should be available again, and your .iso's should play.