Hello,
If you have PFU (Partner Firmware Upgrade) enabled, then yes the firmware will upgrade to which ever controller has the latest firmware version. I would recommend that you install the new (decommisioned) controller into the system wait 30 minutes. This will give the controllers enough time to sync, copy configuration information, and upgrade firwmare. Once that is done I would suggest to upgrade to the latest firmware verion using the Smart Component. Using the Smart Component will also flash the midplane and controllers. This will ensure everything is at the correct version for better operation of the array.
The latest firmware versions can be found here: https://h41111.www4.hpe.com/storage/msafirmware.html
I would also suggest that this would be a good time to upgrade the hard drives in the system. You can find the drive firmware here: https://support.hpe.com/hpsc/doc/public/display?docId=emr_na-c04315770&lang=en-us&cc=us
Cheers,
Shawn
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