Cadalyst Tip Reviewer Brian Benton shares a tip about making new blocks from existing blocks in AutoCAD.
"Many times we need to create a new block that is similar to a block we already have. You can explode an instance of that block and create a new one from the linework. Or you can select an instance of the old block, right-click, and use the menu shortcut command of Copy Block Definition and Assign. Or you could type CopyBlockDefAndAssign on the Command line. Of these choices, I prefer the right-click method.
"This command will create a new block in the file and replace the selected block with the new block. Make sure that the block you selected to copy needs to be the new block. This gives you a starting point in making a new block. It is especially useful when an existing block is similar to what your new block needs to be."