I ran into a dead end with parameter BOINC_DIR
while trying to move the working directory away from /var/lib/boinc-client.
Nothing worked until I turned /var/lib/boinc-client into a symbolic
link to /srv/boinc-client as a work-around fix.

I have installed boinc-client per:
testguy@tug:~$ aptitude versions boinc-client
i 7.6.33+dfsg-12 stable 500
on Debian 9.8.0 Stretch loaded on a 1U blade
that I use mostly as a firewall/router/server at my home.

What follows is copied from /etc/default/boinc-client:
# This file is /etc/default/boinc-client, it is a configuration file
# for the /etc/init.d/boinc-client init script.

# Set this to 1 to enable and to 0 to disable the init script.
ENABLED="1"

# Set this to 1 to enable advanced scheduling of the BOINC core client and
# all its sub-processes (reduces the impact of BOINC on the system's
# performance).
SCHEDULE="1"

# The BOINC core client will be started with the permissions of this user.
BOINC_USER="boinc"

# This is the data directory of the BOINC core client.
# BOINC_DIR="/var/lib/boinc-client"
BOINC_DIR="/srv/boinc-client"

# This is the location of the BOINC core client, that the init script uses.
# If you do not want to use the client program provided by the boinc-client
# package, you can specify here an alternative client program.
#BOINC_CLIENT="/usr/local/bin/boinc"
BOINC_CLIENT="/usr/bin/boinc"

# Here you can specify additional options to pass to the BOINC core client.
# Type 'boinc --help' or 'man boinc' for a full summary of allowed options.
#BOINC_OPTS=""
BOINC_OPTS="--allow_remote_gui_rpc"

# End

I see where the above is read into the /etc/init.d/boinc-client script.
I find that the other settings do work.
When the change did not happen, I then edited the boinc-client script default
setting to use the desired place on /srv/ directory. That failed too.

Note that
1) command: "...# journalctl -xe" and
2) command: "...# systemctl status boinc-client"
both show that boinc is stuck in "/var/lib/boinc-client" directory.

Before posting this I searched without finding any similar report.
Therefore if someone else can duplicate my work with BOINC_DIR
then it might be a bug in the /etc/init.d/boinc-client script.
Please note that my desire to move the directory stems from the backup
methods in place on the host and are not related to boinc itself.

Thanks in advance for any feedback.

# This file is /etc/default/boinc-client, it is a configuration file
# for the /etc/init.d/boinc-client init script.

Note that
1) command: "...# journalctl -xe" and
2) command: "...# systemctl status boinc-client"
both show that boinc is stuck in "/var/lib/boinc-client" directory.

You're starting boinc through systemd so the above file is irrelevant. /lib/systemd/system/boinc-client.service seems to be the place to look. Don't edit that file in place but find out how to properly override the settings.