Transferring from C Drive to D Drive

Unless Origin has something newer now, this was the normal way of doing it some time back:

Step By Step Guide Below (found here)
This is written with the assumption you want to reinstall origin after a windows reinstall, however it works for all of the above scenarios.

1. Find your Origin Games install folder. This is found under Gear Button > Settings > Downloaded Games.For most users, this will be "C:\Program Files\Origin Games"

2. Close down Origin (if its open), Navigate to the folder, and back up all of the game installs In the folder you will find subfolders that are named after all of your games (i.e. "Battlefield 3" ) - copy or move these to the desktop (or another easy to find place) - this may take a while depending on your setup, my games were ~60gb.

3. Uninstall/Delete current Origin install. You'll then need to get rid of your current origin setup. If you're moving from one drive to another, you'll need to first uninstall origin, then delete its install folder. Make sure to select "No" if it asks if you want to keep its settings. If you're reinstalling origin after a windows reinstall, you wont need to do this and can just delete the Origin folder.

4. Reinstall Origin. The natural next step is to reinstall origin in its desired location. It really doenst matter where you put it! Make sure that you've ran the program and logged in, and make sure the games install directory is where you want it to be ( again this is in the settings)

5. Initiate a download for your games. Don't worry! we're not really going to redownload all of your games. This next step makes origin do all of the registry linking and whatnot. Make sure the download has got to at least 0.01% or had created the folder for the game in the origin games folder.

6. Close down Origin, and move your games back. leave the downloads running, close down Origin in the tray and then proceed to move your games that you backed up earlier to the desktop (or wherever) back to the games install directory. When prompted to "Merge Folders" click yes, and select "Move and Replace" if windows asks to overwrite any files.

7. Open Origin up, and install the games! Open Origin back up, and after giving it a few seconds, it should start to read the games. After a few more seconds, it'll tell you it's ready to install! click "install", and you're ready to go!