AutoCad :: Changing Line Weights / Layout Space To Make Changes Permanent

I need changing the line weights.I was given an AutoCAD 2009 file that already had the drawing created. In the Model space, the drawing has been created on Layer 0. Some of the other lines have a different layer - but most of it is created on Layer 0.My manager has asked me to simply make some of the lines bold and leave the rest as is.

When I turn on the LWT button at the bottom of my program, the line weight is extremely bold (shows "By Layer" for the line weight). I have gone back to each layer to select a thinner line weight and saved the drawing. However, when I go to print, the line weight defaults back to the “By Layer” line weight.

do I need to be in Layout space to make these changes permanent?

I have then created a new layer (with a new colour) and selected my preferred line weight – and changed the lines that I want to be thicker, to the new layer I created. When I go to print, I went into my printer settings and insured that the new layer I created and colour has the preferred line weight I want. However, when I preview my drawing, it has made all the other layers thicker, and the ones I want thinner.

Should I follow the previous forums and threads using the pline and pline edit?