From the documentation, it states that the A-Star 32U4 that comes with the pre-assembled Zumo Bot has a Caterina bootloader (https://www.pololu.com/docs/0J63/8).
Why do you want to replace the bootloader on the Zumo 32U4 Robot? We have not tried putting Optiboot on the Zumo 32U4 Robot, but in general it should be possible to get a serial bootloader working on the robot’s ATmega32U4. You would need to unplug the LCD, since the TX and RX pins of the microcontroller are used for LCD control lines. If you try it, please let us know how it goes, and let me know if you have any further questions.