It is possible to change just about any icon that comes with an application.  There are a few different applications that will help make this job easy and my favorite is resource hacker. The reason that I can think of that someone would want to change a programs icon are the following:

• Two programs have icons that look too similar
• The program comes with an icon that does not give a good representation of what the program does
• To have the coolest icons on the block

Resource Hacker
Replace Icon

The steps to replace an icon for a program are fairly simple. Just follow the below steps to modify the icons for any of your applications.  

1. Download Resource Hacker
2. Unzip the file and run ResHacker.exe
3. Open the program that contains an icon you would like to change (like C:\Program Files\Brett Batie\Excel on Multiple Monitors\runExcel.exe)
4. Click Actions→Replace Icon in the Menu Bar
5. Click the “Open file with new icon…” button
6. Select the new icon (like C:\Program Files\Microsoft Office\Office12\EXCEL.exe)
7. Click the “Replace” button
8. Click File→Save in the Menu Bar
9. Close Resource Hacker All Done