Block replacement from selected blocks, will replace blocks and erase old block or retain it, you could add extra symbology to old blocks or replace them. This replaces them from another block definition in the drawing, it does not redefine them. you could replace a block, purge it then rename the replacement block to the old name. It will only replace one scale factor so set it in the dialog.

There are a 14.02 version and a 2000 AutoCAD Visual Basic for Applications project files.