Does porting code to different MCU clock frequency affect serial baud rate?
Yes, it does. As can be seen in the "USART" section of the datasheet the baud rate is generated directly from both fosc (aka FCPU) and the value contained in UBRR. The tables at the end of the section gives the appropriate value for UBRR for a given CPU frequency, desired bit rate, and state of the USART clock doubler.