I was actually referring to a transition from PHP4 to PHP5. As I originally stated; there was not a smooth transition - PHP4 is almost nothing like PHP5. So, what I really meant; was that the difference between the two is quite stark.
I'm not quite sure what you mean about the transition? I can certainly say it has been better than the one from PHP3 to PHP4 which taught the hosting companies a big lesson! Regards Marco