Wu YiFei,
There is no tool that I know of. What I did, which happens to be
unsupported, is to create a 2002 solution & project.

Open the 2002 & 2003 .sln files in notepad, manually change the 2003 .sln to
match the version info from the 2002 file.

Open the 2002 & 2003 .csproj files in notepad, manually change the 2003
.csproj to match the version info from the 2003 file.

If your project is simply enough, you could just create a 2002 project and
add all the files from your 2003 project to it. Using the 'Link File'
(dropdown on the side of the open button) in the add existing item dialog.

Hope this helps
Jay