By upgrade, are you referring to the software or a physical replacement of the audio head unit?
The latest software version is 59.00.502 on the MZD system, but I don't know offhand whether that also applies to the MK1's TomTom based system as well. It's probably worth checking it with the dealer about getting it updated if necessary. I know there were some issues a while back with usability of the USB audio files, but I think that was resolved with a software update.
However... I do use the bluetooth audio but only for Spotify. You can only pause, skip forward and skip back with the car's controls. Everything else has to be done from the phone screen itself and the album artwork doesn't display on the car screen either. That's why I have a charging cradle on the A pillar so I can see the Spotify screen. I would imagine that using stored audio from a phone may be the same? If you use the inbuilt Aha feature you can switch between menus with that using the car controller to select different channels, podcasts, etc.
I think someone did raise the subject of swapping the whole unit for another one earlier on the forum but can't find it at the moment.
Alan