They don't have to rewrite the rules at all. Current league rules require both teams, in their entirety, to be on the sidelines, standing, during the national anthem. Penalties listed can range from on-field penalties to individual/team fines, to forfeiture of draft picks.
They just CHOSE to not enforce those rules, thinking that their core fan base (which, based on the superbowl ads every year are the beer-drinking, troop-respecting, flag-waving salt-of-the-earth American, not the ***** coastal elites) would support them forever. Reality has slapped them upside the head, and they are trying to reverse course. I doubt it will work...