Before doing a version upgrade on an HMC, make sure that the Machine Type-Model and Serial Number reported in the HMC GUI matches that which is printed on the sticker on the front of the machine. If it does not match, then per IBM do not attempt an upgrade.
While working on HMCs today I found that their two CR8s looked like the screenshot below, which shows incorrect values for these items. I did apply the latest SP for 8.8.6, but I did not upgrade to 9.1.
Physical HMCs will have a Model Type of 7042-CRn (x86-based) or 7063-CR1 (POWER-based) and a serial number in the form nnxxxxx.
Virtual HMCs will have a Model Type in the form Vxxx-xxx and a hexadecimal serial number.
IBM’s documentation states that a BIOS flash will be necessary to fix this condition before the HMC software can be upgraded. To order a BIOS update, you will need to open a hardware service call.