It's very simple to update your version of boinc. Just download the file, once you open the installer you get a few prompts asking where you want it installed etc. The installer will pick up your existing boinc installation, stop the tasks, uninstall the old client, install the new one and then resume your old tasks from where you left off. If you look in the log there will be a message saying version change 6.4.5 to 6.6.31 All your settings and wu's will be the same as your old version as boinc saves your data to a different directory to the client installation files.

According to the CA's its probably not worth updating your installation of boinc as there are some issues with versions 6.6.xx
Having said that I've been running 6.6.20 without any problems for a while now so its entirely up to you.

Thanks for informations.

As far as I've no problems with the running version, and there seems to be no advantage for the new version in terms of speed or quantity of WUs processed, I'll keep the old one crunching...