What is SP-UHC?

Single Payer - Universal Health Care. He's pointing out (correctly) that there's some moral hazard if a state moved ahead with SP-UHC on its own, because you'll get some adverse selection as sick people might move to the state. But a lot of that can be controlled with eligibility and enrollment limits, and IIRC that didn't end up being much of a factor in killing the SP-UHC efforts in the states that tried it. It was just the plain old cost of treating all the existing un- and under-insured in the state.

Would we have to have an amendment to try single payer?

Nope. The federal government could tomorrow decide to offer a national health insurance program that covered everyone in the country.

The obstacle to doing so isn't federal power. It's that switching would significantly increase the amount we collectively pay for health care, and that's politically very difficult.