Only just seen this thread so apologies for replying now and apologies if I haven't ready everything carefully.

It's as Frances says.
I can log on to my account at xara.com and get my serial number for Eye Candy 4000, then enter that to upgrade to Eye Candy 6 for $99.

I haven't purchased it though.
But I could go as far as the web site telling me that it would be $99.

So, as far as I can see, all you need is the serial number if you purchased the full version of Eye Candy 4000.