One word - don't. I have just done the upgrade myself (from 4.8.4 to 4.9.0) to find that a lot of functionality has changed - core underlying cache module (and individual per-module cache controls), some URL rewriting stuff has been fixed (I am writing a URL rewriter for DotNetNuke, but in the meantime am using the free Friendly URL Rewriter by Bruce Chapman of iFinity Solutions, OZ.
The new default skin is much nicer though - certainly helps with inspiring one's own ideas. Speaking of which, what are your thoughts on this design I've used? I've customised it a little from the original but I think the finished product is one that I'm fairly proud of (with a little help from iStockPhoto).

For those people who want to ask how I "smoothly" upgraded, here's a quick overview:

• I was running DotNetNuke 4.8.2 install version (no source code and no manual builds) prior to really adding content or uploading my skin, so a backup wasn't essential, but on commercial projects I've done before with DotNetNuke, always take database and file system backups. One day they will save your life.
• Once you've backed up your site, grab the Upgrade package from DotNetNuke, and unzip it straight over the top of your old installation.
• Open your site up in a browser and it should automatically take you to http://www.yourdnnsite.com/Install/InstallWizard.aspx?mode=upgrade. This page runs the SQL scripts to upgrade the database structures in line with the code base.
• Once done, return to your new site and start playing with the new features!

I have a number of new projects in the pipeline, as well as possibly moving back to the South West to follow up on some opportunities in this region and to be closer to my family.

Stay tuned for more updates!