When starting up a VirtualBox VM I received the following error:
In order to change the VRDP port I would need to start up the VM, but in order for the VM to start up from the GUI, I needed the VRDP port changed. Bit of a Catch-22.
The solution is to start in headless mode from the command line:
VBoxHeadless -startvm BrokenVMName -p 7788
Then power off the VM from there:
VBoxManage controlvm BrokenVMName poweroff
Once it’s powered off, modify the VRDP port:
VBoxManage modifyvm BrokenVMName -vrdpport 4777
Once this is complete the VM can be started as normal.