If I understand your question, you need to change the working units AND the actual size of the drawing in the file. Since you already have the working units down, change this first (to correspond with the new scale).
Next, you need to change the size of the elements in the drawing so that when you measure these objects using the new working units, they correspond with what is shown by the dimensioning.
To physically change the scale of the drawing from 1/8 scale to 3/32 scale, I would use the "element selection" tool to select the drawing you need to change and then use the "scale" tool so that you scale it down to 0.75 of it's original size (I got 0.75 by dividing 3/32 by 1/8). If you want to keep the text the same size, you will have to manipulate the text (by increasing it back to it's original size) after changing the size of the drawing.
Another way of doing this, assuming everything on a certain sheet is the same scale, is just to change the size of the border.
Is this answering your question at all? Sorry if I'm spelling out stuff you already know or if I'm totally misunderstanding you. Good luck, and let me know if this helps....