From at technical perspective? Replace them with Android devices!
Assuming thats not a viable option, we'll need more information on your use case. Are the iPhones already in use? Are they going to be used in a BYOD scenario or for something else?
Department we are migrating gives users the option for Android / iPhones. Some people prefer one vs the other. Devices are already in use so will need to be factory reset. Devices will not be BYOD. I'm feeling for the 40/45% iPhone users it's is going to just have to be the long arduous process. Providing s/n to vendor, having them enrolled and have all users reconfigure their devices.