Steve,
The main reason that there are upgrades & Full versions are three fold...
One, yes to save some money for owners of the legacy OS...
Two, "shot, forgot what I was thinking for the 2nd reason"...
But the biggest that isn't really known is. .
To make installing the newest version easier, over a legacy OS, your older computer's chip set or peripherals (sp?) may not be compatible with the latest OS. Time and Time again I've seen this, even though they say it is..... if one is to have a try at making a full, new, from scratch install work, 1st get a list of all the periphirals in the machine and revisions of them, get to know what chips sets you have. Visit the manufacturers' website for each and see if you can find the new drivers for ME. They may not be fully compatible with a full version of ME.
I would advise against it, try the upgrade... but unless you do all your home work... stick with the present OS or just try the upgrade.
If you choose to take this mission. .
Good luck, tape will self destruct in 10 seconds...
9,8,7,.....
Bob