CMC Module is hot pluggable. All setting and configuration on active CMC will be synchronized to standby CMC. You need to maintain same firmware version on both CMC. Do you know the firmware version on CMC installed on the chassis and one you have. If CMC firmware differ then CMC will be shown as redundant degraded and we need to update CMC FW to ensure both are on same version. 

Refer below link for more details

https://www.dell.com/support/manuals/us/en/19/poweredge-vrtx/cmcvrtx33ug/understanding-redundant-cmc-environment?guid=guid-ae8daa15-8c5b-49ea-bdf9-148c6e6a89f1&lang=en-us 

Hi Shine,

Thanks for your answer. Do you know if we need to power down all the blades/hosts to upgrade the CMC firmware? I have read 2 different Dell documents, in one of them says "if the blades are powered on, it is not necessary to power off", but in a second one says "before updating any chassis components, all server nodes must be powered down"

Not required. There is no need to power off blades for updating CMC. During CMC firmware update process fan will rotate 100% and this is expected

You can get more details below

https://www.dell.com/support/manuals/us/en/04/poweredge-vrtx/cmcvrtx33ug/updating-the-cmc-firmware?guid=guid-9cab13be-7e1e-491b-86ff-c58c86cc0b79&lang=en-us 

Hi Shine,

We have managed to plug the card without having to switch off any blades, and the VRTX has recognised it and it is showing Full Redundancy.

Thanks for your help