Swapping crystals?

I need a uno-ish broad running @ 20MHz to run at 16MHz.

Would changing this in Boards.txt work?

##############################################################

orangutan328p.name=Pololu 3pi robot w/ ATmega328p via Programmer
orangutan328p.upload.using=avrispv2
orangutan328p.upload.maximum_size=32768
orangutan328p.build.mcu=atmega328p
orangutan328p.build.f_cpu=20000000L
orangutan328p.build.core=arduino

to this?

##############################################################

orangutan328p.name=Pololu 3pi robot w/ ATmega328p via Programmer
orangutan328p.upload.using=avrispv2
orangutan328p.upload.maximum_size=32768
orangutan328p.build.mcu=atmega328p
orangutan328p.build.f_cpu=16000000L   // <<<< edited here
orangutan328p.build.core=arduino

Assuming the crystal is changed to match, yes.

You need to change the boot loader as well.