INSERT or CLASSICINSERT are the codes to use (AutoCAD 2020)
Look for your prior drawing in "Examine" and make sure it's inserted without scale or rotation in 0.0 by cancelling the associated selections.
Select the dimension line you want to change with the Select Objects tool, then click the Open Object edit option.
The following is displayed on the Secondary Format screen of the Dimension Line Specification dialogue:
You can now see that the dimension has been changed to reflect both metric and imperial units.
-DWGNUNITS is the command to use (note the " - " in front of the command) This command is "undocumented." After you've decided on Imperial or Metric Units, type MEASUREMENT into the command prompt (note: NOT "MEASURE"). The Imperial ACAD is used when the value is 0 (zero).
Create a new blank drawing in AutoCAD Architecture or AutoCAD MEP.
To bring up the "Drawing Setup" dialogue box, type UNITS.
Change "Units" to "Millimeters" or "Meters" once there.
In the bottom left of the dialogue box, check the box next to "Save as Default."