If you wish to advance the 50$ robot microcontroller, lets say to get more I/O pins. Can you simply put more pins on the board and solder them on differently, or do you need to resolder every part on the board then, to make them adjust to the need modification? And does numbers of I/O pins have to do anything about what ATmega you choose? - So you need a more powerfull ATmega for more I/O pins?