Document units will not convert from mm to pixels

Changing the units for in the Preferences changes the units for all open documents and for future documents without units (until a document is opened that has a different unit).

Changing the units for a specific document can best be done by right or option clicking in the rulers or by changing it in the File > Document Setup.

The units preferences never changed the current document. To change the current document, turn on the rulers and then rightclick into them.