As far as I know, you need to uninstall the disc version and install the download one, yes. It's worth a try and no, you won't loose anything.
Make sure to make a backup of the folder in documents\electronicarts\thesims 3 (that's all your savegames and downloads and what else)
Here's a few tips:
You can then download the game from Origin and install that version. I'm not very Origin savy so I can't tell you exactly where that is and since it's late here, I'm calling it a night.
If you need more assistance I'm back online tomorrow (but remember I'm just a user like you and have nothing to do with EA).