When upgrading the EX switch from software release 9.0Rx to 9.1Rx or above, you will see boot message errors if you have the "edit virtual-chassis pre-provisioned" setting configured.  The "pre-provisioned" configuration setting was changed between releases.

When upgrading the EX switch software from 9.0 to any release starting from 9.1 or higher, you will see boot error messages.  The messages below only happen if the "edit virtual-chassis pre-provisioned" setting is configured. 

With the Virtual Chassis (VC) configuration setting "edit virtual-chassis pre-provisioned" configured you will see boot error messages displayed (see display below).

Boot error messages:

========================================================
Creating initial configuration.../config/juniper.conf:351:(19) syntax
error at 'pre-provisioned'                                                        
[edit virtual-chassis]
'pre-provisioned;'
syntax error
Virtual Chassis Control Protocol: [edit virtual-chassis]
Virtual Chassis Control Protocol: 'member 0'
Virtual Chassis Control Protocol: Cant have serial number
configuration with preprovisioned not set
mgd: error: configuration check-out failed
========================================================

The messages are generated because of a parameter change between software release 9.0 to 9.1 and above.  The "edit virtual-chassis pre-provisioned" setting was changed to, "edit virtual-chassis preprovisioned", where the hypen was removed.

Software release 9.0:         

"set virtual-chassis pre-provisioned".

Software release 9.1 and above:

" set virtual-chassis preprovisioned".

You need to configure the 9.1 command "preprovisioned" after the upgrade is completed.