Never doubt that teams have contingency plans—this is a business, and a large portion of that business is set on image management. Within 24 hours of the internet going bonkers because the NFL was discussing the viability of the Chargers in LA, Mike Florio of ProFootballTalk released a sobering article articulating exactly why the team would stay the course.
The details of the agreement between the Chargers and the Rams are not public, but they aren’t private either. The concept first had to be voted on and approved by the NFL, and the nitty gritty details were worked out between the teams before the move was finalized in January of 2016.