Re: Switching to Steam - Can I still use Store content I bought?

@username12333  You can use your existing Store content in a Steam install.  You'll need to register at least the base game product code you receive from Steam, or else the Store won't know that the Steam install of Sims 3 belongs to you.  But you should get 1000 SimPoints for the base game plus another 1000 for World Adventures, so it's convenient to do so anyway.  You do not need to link your Steam and EA accounts.

You'll also need to download and install fresh copies of all your Store items.  I'd suggest starting with a clean Sims 3 folder, installing your Store items, then moving over whatever you want to keep from the old folder.  But if that's too much hassle, at least delete all the .ebc files in DCCache, plus everything in DCBackup including ccmerged.package—you'll want the game to create a fresh ccmerged as you reinstall the Store content.