You need to go create a mm start template file foe all future files to be in mm.
In config.pro, define the start_part_template to be your new file.
To change the units of the current part file, File-Prepare-Model Properties.
The second entry is for the units of your file. Switch it to mmks or define your own metric units. Save this file (with no geometry) as your start_part for metric parts.
