If you're moving to 1TB drives, simply unplug the 500gb drive, wait for it to disappear out of "Computer", then plug in the 1TB drive.
After that, right click "Computer", then click manage. On the left pane, go to Storage->Disk management. Right click on your J: drive, and choose "Change drive letter and paths". From there you will be able to change your drive letter for that specific drive, but make sure that you're not picking a letter that already exists.
If you plan on having both drives plugged in at the same time, you cannot have them on the same letter unless they're connected to a raid controller, and are turned into 1 logical drive before windows sees them, but that's not what you want to do.
Hope this helps!