Try this:
- Enable Airplane mode
- Connect your phone to your computer and go to the update tab in Zune
- Wait a certain period while it says "checking for updates" (I counted 25 seconds since my internet is rather slow) then disconect your internet by disabling wifi on your computer or removing the network cable
- Zune should say an update is available for your phone. If it displays something else, repeat steps 2 and 3 but experiment with the waiting time. This is the tricky part.
- Re-enable your internet and proceed with the update.
- You might have to go through steps 2-5 about three times as you install a series of updates eventually leading to Windows 7.8.
Now excuse me, I need to experiment with my resizable live tiles! Yey!
Note: I'm using a Lumia 610 that's not locked to either Smart, Globe, or Sun. I think the network doesn't really matter with this method.