It's all fine and dandy to change the dimstyle to a different unit, but when you list that 1" line, it's going to tell you it's a 1" line, even if your dimension says 25.4mm. In order to have the drawing diplay the correct units, you must change the units to decimal. Once you change the units to decimal, you now have a line that is 1.0 long, which would imply that it is 1mm long if you are drawing in mm. You have to scale the line up 25.4 times for it to display as 25.4 units. 1 unit is 1 unit - it could be a mm it could be a mile, it just depends on what you are wanting to show. The help file even tells you this. You have to decide what unit of measurement your unit is going to be.
What you are describing is simply showing alternate units in the dimensions. He wants to convert the exist drawing from inches to mm, not just show alternate mm in his dimensions. The only way to convert the drawing is to change the units of the drawing from architectural to decimal. Otherwise how would you draw something that is 70mm long? If you haven't changed the drawing units, and you draw a line with a length of 70, its going to be 70 inches. How are you going to draw it as 70mm? I guess you could calculate how many inches a 70mm line is, and draw it like that, but that's just dumb, and a lot of work.
If you go into the dimstyle, and set the primary units to decimal, all of the dimension will be wrong unless you change the drawings units and scale the drawing. If you notice on the alternate units tab there is a multiplier in there for the alternate units. It's not magically changing everything, it's multiplying the current unit by 25.4 in order to display it as a mm.