@declan2777 Which method of upgrade you are goint to perform, is it ISSU. The latest release notes (R6616) has the following restriction if you are performing ISSU.
"ISSU is not supported on an IRF fabric that uses 40-GE QSFP+ cables for IRF link connections. To perform ISSU on the IRF fabric, replace the cables with 100-GE or 40-GE transceiver modules or cables"
I personally recommend upgrading the IRF using CLI/console where you will have full control and will know as what is happening, rather than through IMC even though it supports. In eitehr ways, schedule a maintenance window and perform the upgrade.