Hi,
As per the following link you just need to be sure the new switch on factory-default and power off the switch/ connect/ power on
http://www.juniper.net/techpubs/en_US/junos12.2/topics/task/configuration/virtual-chassis-ex3300-member-adding-cli.html
Mohamed Elhariry
JNCIE-M/T # 1059, CCNP & CCIP
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