Its quite easy (at least in the days when I use post nuke). Just install postnuke over you current php nuke, it will get to a stage where it as if its a fresh install or an upgrade, select upgrade, it will then ask you what you want to upgrade from.
WARNING. This was the way it was about 2-3 years ago, it much has changed now, please read the documentation to see if its still possible to upgrade this way I could be wrong.
In the days when I was a big post nuke fan I even register this domain name and provides post nuke into:
www.postnuke.org.uk 