You should have a backup before upgrading! Please ensure that everything works properly on your staging site before pushing these changes to your live site.

There is some breakage in this release. Some actions have been removed, and all frontend templates have changed. Some functionality has been removed completely, though most has new ways of accompishing the same result.

All frontend templates have changed. You will probably need to restyle your templates. Also, if you are calling FEU with parameters to specify templates, you will need to begin using {cms_module_hint} or other similar plugins.

Since the authentication storage mechanism now uses cookies formatted differently, any users that were logged in, or had long term cookies (rememberme functionality) will be forced to re-login. Though their information will not have changed.

Settings and options have changed. You should grab screenshots of the settings pages before upgrade so that you can know how to adjust config entries and templates to the new methodologies.

The SelfRegistration module will no longer work with the v3.0 series of FrontEndUsers see the CGFEURegister module.