Actually, if you have a S3 and you get the new pen, you can easily customize the pen settings for the top button. Your Surface app won't show anything other than pen sensitivity settings until you do what I'm going to tell you. Once you do this, the pen customization settings will appear in your Surface app.
You have to download the new pen drivers from MS. They will show as SP4 drivers in a zip file. Download the zip file and unzip it. Then un-pair your new pen. Then go into device manager where you'll find 3 drivers for the pen. Manually uninstall each driver. Go to your unzipped driver folder you downloaded. Find these three (SurfacePen, SurfacePenPairing, SurfacePenSettingsDriver). Select the first pen driver (.inf) and right-click on it and select install. Do that will all three pen drivers. Once that's done, reboot your S3. Then pair your new pen. Then go into the Surface app and you'll find the customization setting showing for the top pen button (the eraser button). Customize the top button as you prefer.
This works flawlessly as I found this procedure on another forum and installed the new pen drivers to my S3 yesterday. Good hunting!