Re: Sorry, I don't get the shutdown
If you've just been using Ubuntu One for syncing files, then, yes, you can simply uninstall Ubuntu One. The files are already on your computer.
However, it's possible to have stuff on the servers that you don't have on your computer; for example, if you've been using it for backup storage or storing music files. In that case, you'd need to make sure you get your files in time.
Please, people, remember to BACKUP before you install that new system. Same if you're upgrading.