Upgrade/Downgrade the Junos SW Version to match the stack you’re adding to.
Obtain the Serial Number of the new chassis being installed.
CLI: show chassis hardware
Mount the new chassis to the rack and power it up.
DO NOT connect VC Cables yet.
Login to the stack that is in production and set the new chassis’ Serial Number to its corresponding Member # and as a “line-card”. (Example: if this is for a current stack of 3, then the new member will become Member 3)
[edit virtual-chassis] # set member x serial-number abc123 role line-card
Connect new chassis to the stack via VC cables in back.
Verify that the new chassis is present and that it indicates that it’s a “line-card” LC using CLI “ show virtual chassis”.