Ah! Many configuration changes has added between V11.x to V13.x. Hope you have gone through release notes. If not, no worries.
Options 1(
Not Recommended
)- Make Active unit out of device sync group, So both will become Active-Active. In that case easily you can make one unit offline and install image & hotfix(v13.x) and later add in device group. But its tedious process and outage time will more then expected.
Option 2(
Better Approach
)--> Go to 11.x to 12.x in different partition and validate the configuration(i.e Virtual, Pool, Interface, Memory, cpu,etc )in both unit(Active & Standby). If all is well then boot to V13.x partition.
Hope issue will solved.
Hi Expert, Please add your inputs.