For some reason it was really hard to find simple information on changing the Plymouth Splash in Ubuntu 16.04…
Lots of the configuration files listed in articles online don’t exist in my install of Ubuntu 16.04.
The way to update the splash is to run
sudo update-alternatives --config default.plymouth
Select the theme you want by pressing the number to the left of your choice.
Once selected, you will need to run
sudo update-initramfs -u to update your initramfs image