Increase the size of a VirtualBox hard drive

To change the size of a disk, you need to clone it to vdi, increase the size, and then

VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
VBoxManage modifyhd "cloned.vdi" --resize 51200
VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk

To change the UUID of the disk, simply run this command:
VBoxManage internalcommands sethduuid disk2.vdi

Version context

This procedure describes the software and the environment available at the time of its publication. The packages, interfaces, and repositories mentioned may have changed or disappeared; first check the system version and its support level.

cat /etc/os-release
uname -r

Work on a backup or a snapshot, note every modified file, and prepare for rollback. For a current workstation or server, prefer signed packages from officially supported repositories.

Reference: official Ubuntu lifecycle.