Recently, I tried to upgrade the installed port on my FreeBSD:

portmaster -Da

and I got the following message:

===>>> Launching child to update xz-4.999.9_1 to xz-5.0.0

===>>> Port directory: /usr/ports/archivers/xz

        ===>>> This port is marked IGNORE
        ===>>> is already in the base system

        ===>>> If you are sure you can build it, remove the
               IGNORE line in the Makefile and try again.

===>>> Update for xz-4.999.9_1 failed
===>>> Aborting update


===>>> You can restart from the point of failure with this command line:
       portmaster  ports-mgmt/portmaster

I google it and someone suggested to do the following:

Just run a

portmaster -e xz-4.999.9_1

And say yes to the prompt. 

Don’t do it!
I tried that and the nightmare began. It will first ask you whether you want to remove xz. Then it will go through every package that depends on xz, and ask you whether you want to delete them or not. If you accidentally click yes, your system will get into trouble.

Instead, try to deinstall it from port:

cd /usr/ports/archivers/xz
sudo make deinstall

This is a lot safer.