Greg Vice, a piping and instrument designer, sent in a tip that can help you edit dimensions in AutoCAD.
"I have found that the Trim and Extend commands work with dimensions. As a simple example, you can draw a line across a dimension, then use it as the cutting edge."
Notes from Cadalyst Tip Patrol: Editing dimensions can be a pain. As this tip notes, you can shorten them with the Trim command, or extend them with the Extend command. Using Trim will move the nearest extension line to the trimming line. The same thing happens when you use the Extend command.