My work is only with 2D. I would like to reassign shift then middle button's default 3d orbit with the middle button then shift's orthogonal pan. Please what do I put in the CUI (or elsewhere) to effect this change?

Isn't it Ctrl+Shift+Click (Button 3) that invokes the transparent orbit using a mouse?

Sorry, since the version is part of the registration I forgot to post it. I'm using AutoCAD 2015. Not LT, Mechanical, Inventor, just plain 2015.  In the version I have you have to use the shift button AFTER clicking with the middle button to get an orthogonal scan which I have frequent need for. Otherwise you get 3D orbit which is worse than useless when working with 2D drawings. Ctrl+Shift+Click might also do 3D orbit but I have had no reason to look for ways of orbiting so I don't know.

How do you do this in AutoCad 2013?

Do which exactly? 3D orbit or orthogonal scan?I think the controls are the same. Try it and see. To rotate the drawing in 3D around the center of the screen hold down the shift key THEN hold down the middle button and move the mouse to rotate. For orthogonal pan (left/right or up/down, no angles) you hold down the middle button THEN hold down the shift button and move in one of the 4 cardinal directions.