Back everything up you don't want to lose to an external HD. When you upgrade to 10 you have and option to reformat your HD or keep all your files. If you keep all your files Windows will make a folder called windows.old on your C drive incase you want to go back. In that windows.old folder will be all the files you currently have on your computer like pictures folder, documents, music and videos. You would have to choose " Keep Your Files" option. Or reformat in which you would have to re-install all your programs and lose all your files.
If your Windows key is good you shouldn't have to do anything. It will automatically activate. If it doesn't try your key again. I bought a windows key off of SDKey for $12 and strangely enough I have an Windows 8 laptop that I'm using and I used that key to activate another desktop computer.