What changes to do in `/etc/plymouth/plymouthd.conf` after getting a new pacnew file?

I have the following in /etc/plymouth/plymouthd.conf:

# Distribution defaults. Changes to this file will get overwritten during
# upgrades.
[Daemon]
Theme=manjaro
ShowDelay=0
DeviceTimeout=8

And I have the following in /etc/plymouth/plymouthd.conf.pacnew:

# Administrator customizations go in this file
#[Daemon]
#Theme=manjaro

What changes should I make to the non-pacnew file before removing the pacnew file?

None, you can leave it as it is.

O.k, thank you very much!

While you can leave it as is, there will be the same pacnew every time there’s a Plymouth package update. The distribution defaults are now where they should be, so the values are now redundant. Replace your existing file with the pacnew unless you have any customization you added.