I'VE LOST THE CD BUT I HAVE THE BOX WITH THE MANUAL CD KEY

not for disk versions the disk needs to be in the drive however if your game is registered to your the sims 3 account they auto reg in Origin you would need to download Origin and install it launch it to make sure your game is in the my games tab you use the same email and password for Origin as your sims 3 account they are/get linked

then you uninstall the disk version of the game download your digital version

back up your stuff this way you can then place your saves/saved sims/library lots back into your newly created sims 3 folders

http://sims3.crinrict.com/en/2011/03/faq-what-files-should-i-backup.html

also after uninstalling the disk version use ccleaner they have a free version run that a few times including the registry part  to help with the cleaning up of left over files

free version here make sure to select the free one  https://www.piriform.com/ccleaner/download

http://help.ea.com/en/article/how-to-use-ccleaner-to-fix-pc-issues/

and since you don't say exactly which game you lost the CD too you will only need to uninstall the one you lost compiled list in this link other packs further down in that link

http://help.ea.com/en/article/manually-uninstalling-the-sims-3/