I kicked off the upgrade with a simple:
sudo update-manager -d
and then clicking the "upgrade" button listed in the update manager. The upgrade notes suggest you can do this via gksu but I found I had to use sudo from a terminal command-line.
I ran the upgrade overnight so am not sure how long it actually took as there were user prompts waiting to be answered the next morning.
After upgrading I had only one major problem:
- the Nvidia video driver wasn't working, which I fixed after a lot of hassle,