Yes, you can buy a Single from someone here on the forums, buy a SC Single upgrade, and then send the current Single to BFL as part of the SC upgrade, despite never having purchased the original Single.
BTW, anyone notices how painful the BFL naming scheme is? I like the Jalapeno, cuz it doesn't get confused with any other product. But a Single and a SC Single can get confused, and a Mini Rig and a SC Mini Rig also get confused. At least for me...
They went from Single and Mini Rig to Jalapeno, SC Single, and SC Mini Rig.
IMO, they should have gone from Single and Mini Rig to Jalapeno, Standalone, and Big Rig.
They should have kept the pepper theme
Jalapeno - Spicy and fast
Habanero (SC) - Super Spicy and Super Fast
Naga Jolokia (Mini) - Damn Spicy and Damn Fast