Hi.
I am also trying to change the units from imperial to metric. And not mid-project, but in a new project:
I open a new project and import the settings from a previous project which had imperial units. Now, I followed your advice and as you showed on the snip, I went under the Manage Tab, but I cannot seem to find the Project Units option. Are you using ACAD Plant 3D 2018? If not, can you tell me where can I find this option? I am adding a snip of my Manage tab: - maybe it is under the CUI ?

Thank you and kind regards!
Borut Orel