Establishing residency usually only requires domicile in the state. It certainly doesn't take a year; more like 30 days, usually. In some states, a trailer can be used as a domicile. And some states also treat certain actions, such as registering to vote, as presumption of residency. Generally speaking, it's not hard.
you mean like the current ACA which allows people to buy coverage after being diagnosed (since they have to accept pre-existing conditions?)
The current ACA wants you enrolled before being diagnosed, obviously. The stated goal is to have EVERYONE enrolled, always. So when a diagnosis occurs, you should already be in the program because everyone is. And if they're not at this point, they're degenerate sponges living off society's good will with no sense of honor, self-respect, or personal responsibility.
Assuming that it requires residency, that requires that the sick person quit their job, find a place in the new state to live and work for say a year or two while staying alive with a dire sickness. It's not just a bus ticket that you need to relocate, there are many more significant costs associated with a move.
And even in that case, unless the state is already overcrowded, this is essentially a way to grow the state's residency and thus it's tax base.