I am using an iPhone 5s which is associated to an iCloud account with Find My Phone turned on that for various reasons I won't be able to get the password for. This hasn't really been a problem so far as the iTunes and App Store account has been changed to my account so except for iCloud everything seems to work just fine.
I'm scared to update iOS to version 10 though (it's on 8.3 now) as I'm worried that for whatever reason it might require me to enter the iCloud password after upgrading and leave me with a brick. Any idea if this is the case?