I just upgraded WorldBeatPlanet to PHP-Nuke 6.0. The upgrade went flawlessly.
I’ve got it down to a quick & simple routine. Upload all of the files to a temporary directory, since uploading can take a while. When everything’s up, run the upgrade script, and then move the uploaded files into the live directory. Since copying the files to another directory takes a lot less time than uploading, only a few seconds pass from the time I run the upgrade script until the upgrade is complete with all of the files in place, and there’s little or no downtime. It did take me a few minutes to activate some new modules & rearrange blocks, but the site remains usable during that time.