When you upgrade from and older 300dpi LaserWriter to a 600dpi LaserWriter you may see a couple of issues. One is light printing and the other is kerning.
What you are seeing is the difference between the accuracy of the 300dpi and 600dpi printers.
The 600dpi printer will print a more true representation of a grey value such as 20% because the halftone uses a much finer screen. Compare a 20% grey printed in a newspaper to that of the 300 dpi printer and you will see that the newspaper is much darker. Compare the 600 dpi grey with the same grey as that printed in a book using a printing press and you will notice that the 600 dpi grey will look darker than the more accurate 1200 dpi representation that the final press will give.
Text printed out on a newer 600 dpi printer using built-in fonts (such as Courier) will also appear lighter because the font used in the printer is set to draw at 600dpi. To workaround this, download the truetype version of the same font included with the printer software to the printer. The substituted font should give "darker" results.
The kerning difference is caused by a difference between the versions of the Courier font in the computer and the LaserWriter ROM. Downloading the font to the printer will also help solve this issue.