Deploying a new switch into an existent MIST managed VC could cause the later to disconnect if incorrectly configured.

If the VC or the newer member are miss-configured, there could be a mastership loss and thus, a connectivity outage.

Follow the instructions in the following link:

https://www.juniper.net/documentation/us/en/software/mist/mist-wired/topics/topic-map/modify-vc-preprovisioned.html