Had a couple of issues with the 586 box and the 64-bit system, one package would not install on the 586 which was easily fixed upon a reboot. The 64-bit had major issues. Upgrade became stuck on one particular package which caused the system to pretty much freeze. I was able to manually reboot, chose the recovery mode and repaired the packages, which was an automated process. During which time, it installed the remainder of the new packages (as they had already been downloaded earlier in the process) and upon a reboot, it was on 10.04LTS.
Other than the new font used for the 'buntu series (on the initial splash screen?), the graphics and desktop background image otherwise did not change. There was no xubuntu-lucid desktop wallpaper included, perhaps the maintainers have not yet updated it.
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