Well after considerable indigestion, I took the plunge. I made the appropriate backups, (3000 miles to the datacenter, so an onsite visit was not an option).
I must say, that my installation was about as vanilla as they come (out of the box fedora core2, and plain jane Plesk with no options/upgrades, and un-touched MySQL that shipped with plesk)
I did the upgrade through the update manager in plesk. (I admit that I hovered over the 'are you sure' button for more than a few moments)
The upgrade took about 2 minuites to complete, and went off without a hitch. I then updated everything else that needed updating which also went without a hitch.
My compliments (and thanks) to the Plesk Dev team, although my environment presented little in the way of challenges, but none the less, the upgrade went as smoothly as anything could.
Thanks to all who responded with advice and comments.
J.