Main thing to ensure when planning an upgrade of any OS is to be
prepared for what to do in case of failure. Having a separate Home
partition in Linux helps as even a totally wrecked upgrade can be
recovered from by doing a clean install and using the advanced option
to manually select the partitions from the existing ones. You'd still
need to add the programs you use but that's not such a great hassle
and in the case of Virtual Box all you need to ensure is that the
virtual machine files are saved in your Home partition and you can
pick them up from there.

Other points to watch are making sure all updates are done to the
existing Ubuntu ( should be keeping up with those automatically ) and
if using PPA's then temporarily removing them will help as it's often
these that balk an upgrade. You can always add back after the upgrade
and if a PPA is no longer valid you will just get a failure to install
rather that a trashed upgrade :-)

As a 'belt and braces' approach you could image the entire system onto
an external drive so if it goes totally awry and you cannot get back
then you can restore the image and start all over. It's the approach
I'd always take when considering an upgrade to Windows but have to say
that the times I've ever done upgrades to that OS they've never taken
well and always ended up needing a clean install anyway At least
with Ubuntu and the separate Home partition you have a fighting chance
of it going well !!