You don't need to reflash.
Plug the new drive in the front USB, initialise it, shut down the box, swap the drives so the new one is in the back, restart the box, make sure its mounted as HDD, create the swap file, download the EPG and your done
To initialise the drive its menu - setup - system - storage devices - initialisation
To check its mounted as HDD its menu - info panel - plugins - mount manager
To create the swap file its menu - info panel - plugins - swap manager