I currently have a virtual chassis with two EX4550s where one is the master and the other has the linecard role. What is the best and easiest method for changing the member with the linecard role to the backup role with minimal/no downtime?
You can check this link
preprovisioned configuration from step.7 to step.9
so you can choose the role of each switch based on the serial number. No downtime for user traffic, you might logout from the switch then you can login again after modifying the roles according to your config.