Not necessary... when you download full versions of let's say Mozilla 1.7.1 and Thunderbird 0.7.2, it will warn you while you're installing them that installation will erase all personal files, everything in order to install new version. At that point you should be aware that you have to save your personal Mozilla/Thunderbird/Firefoxfiles somewhere else, and then do with them whatever you want latter...
Cheers !