I did find Juniper doc describing how MASTER SW is elected in VC, however I do not see how the Virtual Chassis behave in following situation:
Existing Virtual Chassis state( using dynamic method i.e no non-provisioned method):
SW1 MASTE RE Priority 140
SW2 BACK UP RE priority 130
New SW 3 with priority 131 is connected to VC, will SW3 become new BACK UP RE and SW2 is demoted to line card role?
Thanks and have a nice weekend!!
In this case it should happen only if you reboot SW2.
It happens because changing role between RE and line card causes some interruptions in service.
Please use check these two links: 1 and 2.