I think you can do that in registry Editor. I had the same Thing before couple of Months, I wanted to Change the unit in active Workspace Editor.
I had to go to: HKEY_CURRENT_USER\Software\Siemens\AW_Retained\version\Common\C\Measurement\
and then add a new DWORD-value and addas value name: Measurement_Display_Units and value_data: 1 (for mm)
AW_retained is for active Workspace, but you can try it or search under HKEY_CURRENT_USER\Software\Siemens
Measurement > Preferences > Calculations tab.
You can see the Units, Model units and Display units.
File > Preferences > Teamcenter Integration.
Under 3D Loader tab, you can see the 'Document Units'