Actually, you are all incorrect.
1. I don't want to save the changes to the layout, as I only want to print a half size plot for checking my work and saving paper. The layout needs to stay as it is, or I have to keep changing it.
The correct answer is: Go to the Page Set Up manager, Select your saved page set up, click the "Modify" button.
Click the "Properties" button next to the Plotter Name, under the "Graphics" heading select "Custom Properties", click on the "Custom Properties" button in the "Access Custom Dialog" below the drop down tree. Then select the paper size/tray on the printer.
But of course, there was no need for AutoCad to make these changes to the Plot dialog box, the 2000 version worked just fine. Changing things for the sake of making changes does not always make things better. Two thumbs down on this version.