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.