Don't change that file, unless you are aware and willling to deal with the consequences.
Quoting directly from that file:
###############################################################################
# FILE AUTOMATICALLY OVERWRITTEN BY PI-HOLE INSTALL/UPDATE PROCEDURE. #
# ANY CHANGES MADE TO THIS FILE AFTER INSTALL WILL BE LOST ON THE NEXT UPDATE #
# #
# CHANGES SHOULD BE MADE IN A SEPARATE CONFIG FILE: #
# /etc/lighttpd/external.conf #
###############################################################################
What version of lighttpd are your running, pimonkey?
Your original description is a bit mangled, mentioning changing lighttpd.conf and external.conf alike.
For lighttpd version 1.4.46 or above, you should be able to overwrite the port from Pi-hole's lighttpd.conf in your external.conf like this:
server.port := 8000
Particularly note the := operator (which isn't supported in older versions of lighttpd ).