of course it can be done. You can always downgrade if you want to, but you will have to make a complete Erase and Install. You cannot downgrade without having to erase your disk.
Think about it as a downgrade from 10.3.9 to a version that came with your machine (ex. 10.3.5). It simply cannot be done without erase and install.
Another thing you can do, is to split your disk into 2 parts, one for panther, and one for Tiger. That can be done, too, but you will also need to format your disk.