I tried to revert to 'Basic'. VBox says that the current stage is still changed. Boot the VM, close it and choose to revert. Still, current state shows as changed. Close VBoxManager, restart. Try to revert. Reboot. Revert. Nothing. Close VBoxManager, relaunch. Big surprise, all the fields in the VM are empty and inaccessible!!! What??? Time to hit the tools.
Close VBoxManager. Open '10.10.vbox' and '10.10.1.vbox' in a text-comparing editor (remember, they were almost identical). Compare the two and below is the "corruption":
10.10 : <AttachedDevice type="HardDisk" ... 10.10.1 : <AttachedDevice type="HardDisk" hotpluggable="true" ... 10.10 : <AttachedDevice passthrough="false" type="DVD" ... 10.10.1 : <AttachedDevice passthrough="false" type="DVD" hotpluggable="true" ...Removing the hotpluggable="true" from the '10.10.1.vbox' fixed the VM. Now, as far as I know there is no way to set a HD/DVD as hotpluggable from the UI and even in the VBoxManage commands, I couldn't find anything relevant. The only brief mention is something in Chapter 9.24 and some mentions in several older release notes.
I'm not sure what happened here, but I'm just posting it so that if someone that stumbles upon it in the future, the search engines will pick it up. Of course I definitely wouldn't mind an actual explanation from anyone...