update-manager --devel-releasein terminal or enter this line in Alt-F2.
There were no errors no hiccups. Because of the relatively slow connection that day download took a while and the whole of upgrading took few hours. There were 1,060 files to download of total size close of 1G. May be it's faster getting iso image from here and do a fresh install.
At the end too bad the sound card isn't recognized by 8.04. It was OK at earlier alphas. My son can't play online games in ubuntu for a while.