Hello all,
As far as I am aware, the Change Primary user option is only available for devices that have been Joined (not registered) to Azure AD or Hybrid Joined devices. Currently, Co-Managed devices are not supported for this feature but Microsoft is known to be working on it.
So, if your device was registered to Azure AD you will most likely need to un-enroll it completely, do the clean up on Azure AD / Intune management portal side of things to make sure no records are left behind, and then Join the device in question to Azure AD.
https://docs.microsoft.com/en-us/azure/active-directory/devices/concept-azure-ad-join
I can confirm that switching primary user works. And it worked for me as in some cases where I did not want to enroll a device that is occasionally being used by more than one user, under DEM or as a Shared Device.
Hope it helps.
Thank you.