To be honest, I don't know whether a Full Retail can be used to Upgrade an unactivated system -but I think it should work.
Back up your data and settings anyhow - using the Windows Easy Transfer process - to external stoage, in case it fails badly.
Make sure that you start the upgrade from within Windows, rather than by booting Windows from the DVD.
If you're lucky, you won't have any problems.
Argh! got it the wrong way around.!
OK - Try again....
You cannot install Home Premium as an Upgrade to Pro - it's a downgrade.
What you could try is using the WET process to export your settings and data to external media, then install the Home Premium in a new Parallells environment, and then import the WET data to that. It should take 90% of the pain out of it, and you still have the trial to fall back on if it goes pear-shaped.