If the application's package doesn't contain an icon file, it will go back to the default application icon, which is a piece of paper with the pencil, ruler, etc.
You can check if there are icon files by CTRL-clicking on the application and choosing "Show Package Contents" then navigating to the Resources folder and looking for .icns files.
A quick hint - before you change the icon on a file, do a CTRL-X to cut the icon. If it is the default icon for that file, nothing will happen. If it changes, then you will lose that icon if you paste a new one on top of it.
You can easily save these icons by creating new folders and copying the old icon onto the folder. You can then copy it back from that folder after you get tired of the new icon.