If you have workflow enabled, this process will not work because you need to update the most recent version of workflow. If you're only updating 3 classes, I'd suggest just updating those 3 specific page types manually.
Now if you want to update the page template for a specific page, again, if you have workflow enabled, it will not work without updating the workflow process. What I'd suggest in this case is creating some C# and use the API to perform the one time update.