When you do the recommended following (or, use the Reset button, or use the GUI to perform a full reset to factory defaults), it resets the firmware installed to a good/known state. In other words, the firmware doesn't change. It simply provides the defaults the firmware is expecting.
I seen this advice repeated over and over. However, even after searching, I can't find any specifics as to what "minimal and manual configuration" means. The closest I've found is: "....just enough customization to connect to your ISP and secure the router's gui and wireless networks you have...
www.snbforums.com
but it’s worth having a look at them all, because you’ll never know when they might come in handy.