Well I have interchanged the following 28 pin DIP AVR chips in a Arduino clone board, meg8, mega168, mega328. All worked well and I was able to reburn bootloaders for all three using a USBtiny programmer and the ISP connection on the Arduino board using AVRDUDE. So either blank chips or chips with pre-burned arduino bootloaders can work fine. Just be sure to change the board selection in the Arduino IDE before compiling or downloading a new sketch.
Lefty