First, you should always be backing up your computer. With a Mac it is super easy with Time Machine. So if you haven't been doing it, shame on you.
I would highly recommend doing a full backup using Time Machine to an external Hard Drive before doing your upgrade(s). Everything SHOULD be there after you upgrade, all your emails and contacts and calendar information, however there is always a chance to loose data and thats where the back up comes in handy.
I am a child of the universe. I will walk gently and true
Of course I'm a creationist. I believe man created god.