If all else fails, you could try to completely re-initialize the Diagram: copy the definition back to where it originally was, modify it in the desired way, and call Init(). The original specification would be in diagDesc[0], and the HTML element where it was stored in diagList[0], so
diagList[0].innerHTML = diagDesc[0];
would restore the definition like it was loaded. You could append a line squareSize=... to the definition, which would overrule an earlier definition of squareSize. For the piece names it is probably easiest to change those afterwards in imag[n] than modifying their original definition.
If all else fails, you could try to completely re-initialize the Diagram: copy the definition back to where it originally was, modify it in the desired way, and call Init(). The original specification would be in diagDesc[0], and the HTML element where it was stored in diagList[0], so
diagList[0].innerHTML = diagDesc[0];
would restore the definition like it was loaded. You could append a line squareSize=... to the definition, which would overrule an earlier definition of squareSize. For the piece names it is probably easiest to change those afterwards in imag[n] than modifying their original definition.