Migration
Do I need to upgrade the iOS in my old iMac before I migrate via time machine to a new iMac?
iMac
No you do not. The migration will only affect (if you have selected them) your preferences, files and user installed apps. Also, nothing is deleted from the old Mac. Incidentally, Macs do not run iOS. That is the operating system for iPhones and iPads. Macs run the MacOS. They are completely different operating systems.