Upgrading Fedora 30 guest to 31 resulted in blank gray screen unusable whether upgrade from GUI or from command line. Rebooting and selecting previous releases at grub produced same results, and the rescue option resulted in empty useless black screen. Nothing stood out in the VBox logs. The only way I could get working Fedora 31 guest installation was to download the iso, boot it in the VirtualBox guest and install it - note, this is not a production guest so didn't matter that it wiped Fedora 30 (same as doing a full new install). The previous upgrade attempts were done on fully up to date Fedora 30 guest no added apps or modifications except removed rhgb and quiet from grub. Didn't have this problem upgrading guest from Fedora 24 thru 30.

Not sure if you have a bug but you can report it.

There has always been a sort of gamble by doing dist upgrades (at least in my opinion).

On the radio today they were selling an online backup plan. The host said something like "in this day and age you have to have a backup".

1 members found this post helpful.

I keep backups of all my vbox guests, so this wasn't a show-stopper, easy to reverse rsync from backups, but always good advise jefro - thanks. I mainly put this up here to see if others experienced same problems.
I'm going to mark this solved although full install over an existing system is the nuclear approach, in this particular case use cause no harm (no data loss, none to lose )