Firefox 3 is so radically different from Firefox 2 then I would advise that you go for a complete fresh download. I may be wrong but I do not think that it will update from within the program.
You can download and install and uninstall as many themes as you want and swap them around under the tool/add-ons/themes menu... nothing "has to be" the default theme.
I have found that some themes,once they are compatible with version 3,no longer work with earlier versions.
Anything on the Mozilla website with a green install button is compatible with Firefox 3.