How are you trying to change the extension of the blind corner base? You should be using the Blind Corner Displacement parameter. See the attached image. Simply change it to the dimension you want and draw the cabinet. Once the cabinet is drawn the parameters are stored in each cabinet part. To retrieve them choose a part and use the Retrieve Cabinet Defaults tool. You can save these defaults to a file using the Save CabWriter Defaults icon and access them later with the Open and Load CabWriter Defaults icon.
See Greg’s video on Blind Corner Cabs at https://www.cabwritersoftware.com/portfolio-item/cabwriter-drawing-blind-corner-cabinets/ . You will find many other videos ar https://www.cabwritersoftware.com/education/cabwriter-training/ .