Hi HendersonD,
The procedure you mentioned is correct.
Make sure you have GRES/NSR/NSB and set system commit synchronize enabled.
Do master RE of EX4500(0) to EX4200(3).
Once EX4200 is new Master.
You can now remove the EX4500 and delete the VCP ports first via cli.
later change the configuration of the virtual chassis so it can be member 3 and 2 (you can leave it like that or renumber it).
Later remove the configuration of member 0.
Note: the VC will stay in mixed mode, until you reboot the entire stack to disable the mixed mode feature (having the member in mixed shouldn't cause any harm)
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Thank you,
Franky