Windows 10 search broken after in place upgrade

I had this scenario last week and it was difficult to find the solution. After an in place upgrade from Windows 10 v1709 to v1809 (using a ConfigMgr in place upgrade task sequence) the search function was broken. I couldn't even type anything to search. I couldn't find anything similar online so I was a little stuck. Although there was an easy fix I needed to be able to apply it programatically.

The fix was to reset the Cortana app but I couldn't do this programatically in the task sequence. 

• the issue only occurred during in place task sequences (1709 -> 1809). It did not happen for bare metal builds.
• it only happened with existing profiles on a machine. New profiles were fine.
• the 1809 wim had the latest SSU and CU.

At first I thought it couldn’t be a group policy problem as it only affected in place upgrades. However then I noticed that it only happened with existing profiles and that made more sense. I tested with an OU blocked for inheritance and the issue didn't occur so I waded through all the customer GPOs.

The issue was solved by reverting these two settings that the customer had disabled: 

Computer Configuration\Administrative Templates\Control Panel\Regional and Language Options\Allow users to enable online speech recognition services

Computer Configuration\Administrative Templates\Windows Components\Search\Allow Cortana

I hope this helps someone else who encounters the same issue. Until next time....