Hi,
As from Junos 11.4, in a mix of EX4500 & EX4200 VC any switch can be configured for any role.
I don't think there are any strict rules for the VC wiring as long as there is adequate resilency/redundancy.
A change in member IDs would require a manual change in the configuration to swap the fpc number around on the interface naming.
Cheers,
Ashvin