is the game regestired in your orign account ? if it is you can look there for your code and or uninstall the disk version from the control panel-programes and features then check for left over keys and files also if you have ccleaner from piriform they have a free version run that a few times to assist with getting rid of the left overs then download your digital version of the game
http://help.ea.com/en/article/the-sims-3-seasons-manual-uninstall/
if you do decide to download the digital version if you run into the DLC alert ignore it and do this to be able to install the download version without reinstalling despite that message, do not click on the download button in origin click on the “i” (information) button this will open another window that also has a download button click on that to install
edit typo