I got the 1 TB fusion drive for my late 2012 iMac (full spec in sig.). Instead of pulling the trigger for a new retina iMac (can't really justify the cost yet...) I'm curious about the possibilities to upgrade my iMac. Is it possible to replace the 128 GB SSD that comes with the fusion drive to another SSD, let's say a 512 GB version? And still use the 1 TB HDD, so that I would have a 1.5 TB Fusion Drive? Thank you for your response.