You could look at refactoring tools such as Resharper from JetBrains or Refactor! from Dev Express as these tools enable you rename something logically throughout a project. A few downsides: won't work with Express, they are commercial products, and they may not work directly with L2S diagrams (although you can rename your column through code first to update all references, and then manually update just the diagram).
You can also use Visual Studio's Refactor command. Right click the property name in some code block (e.g. where you're using something like myProduct.ProductNumber right-click on ProductNumber) and then choose Refactor | Rename. This will rename all occurrences for that class *except* the one in the .dbml file which you need to update manually. Not sure if this command is available in the Express edition.
Finally, you could have used Search and Replace. For something like ProductNumber this would have worked fine I think. But yes, you still need to be careful and make sure you're not renaming the wrong things. Once you get a bit more experienced in this renaming, you'll be able to do it in minutes, rather than hours.