A State changing how it runs its State Legislature is a change to the State Constitution either way, whether taking away the map drawing authority from the Legislature itself or changing the structure of the Legislature to be non-districted or multi-member districted.

What is more likely, a change to federal law, or the existence of one of 50 states that changes state law?

It’s a single statute that requires single member districts. It wouldn’t be difficult to change. Hell, it may even be unconstitutional.

> It’s a single statute that requires single member districts. It wouldn’t be difficult to change.

Yes, it would, politically. The reason it exists will produce vet strong resistance to a straight repeal, and any replacement with an alternate approach to preventing the original problem while allowing multimember districts will, in addition to resistance from those in Congress whose opposition is driven by opposing undermining the existing dominant parties, resistance around the details of any alternative scheme (which will have predictable winners and losers.)