Units in acad are really neutral 'cartesian' units which are "converted" to
other units systems for coordinate entry or annotation purposes.
DDUNITS used to get you to the dialog that sets your units and accuracy.
Maybe that still works. There is probably a menu item if you are using
vanilla menu that's called "units" or somesuch.....
If you've already drawn things under one system and NOW want to change
systems, don't expect acad to know to treat these things as if they had
"real" dimensions. One meter could convert to one inch, and you may have to
rescale your drawings, as well as make other changes to dimensions and text
styles, not to mention system variables.
Click to see the full signature.