@Paul, I did make changes to both files (config.cfg and emoncms-export.sh. My intention was to automate the backup process Emoncms automatic data backup from EmonPi to Remote Computer.
I changed the filename date format in emoncms-export.sh and the backup location in config.cfg. I have not been able to backup to a remote server (yet) but I am auto-backing-up to a USB drive for now. I wanted to backup to the server so that, in turn, that would be backed up to Time Machine and I would have backups going back for years then, without intervention from me. Perhaps I’m being too paranoid though 
I am surprised that changing these files would cause an update failure. But there may be reasons beyond my knowledge.
Log rotation seems to be working properly now, /var/log is not filling up.
I am available for some testing or documentation for any issues that arise so please feel free to ask.
pi@emonpi:~/backup $ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 3.9G 1.7G 2.1G 45% /
devtmpfs 484M 0 484M 0% /dev
tmpfs 489M 0 489M 0% /dev/shm
tmpfs 489M 32M 458M 7% /run
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 489M 0 489M 0% /sys/fs/cgroup
tmpfs 30M 24K 30M 1% /tmp
tmpfs 50M 8.1M 42M 17% /var/log
tmpfs 1.0M 0 1.0M 0% /var/tmp
/dev/mmcblk0p3 11G 162M 9.7G 2% /home/pi/data
/dev/mmcblk0p1 43M 22M 21M 52% /boot
/dev/sda2 29G 75M 28G 1% /mnt/usbdrive
tmpfs 98M 0 98M 0% /run/user/1000
and …
pi@emonpi:~/backup $ ls -la /var/log
total 3600
drwxr-xr-x 12 root root 580 May 23 16:17 .
drwxr-xr-x 12 root root 4096 May 18 07:05 ..
drwxr-xr-x 2 root adm 120 May 24 05:17 apache2
-rw-r----- 1 root adm 152315 May 24 07:17 auth.log
-rw-r--r-- 1 root root 6758 May 22 08:39 boot.log
-rw------- 1 root utmp 0 May 22 08:39 btmp
-rw-r----- 1 root adm 419427 May 24 07:17 daemon.log
-rw-r----- 1 root adm 1061241 May 23 16:17 daemon.log.1
-rw-r----- 1 root adm 1265 May 22 08:39 debug
-rw-rw-rw- 1 root root 2563 May 23 23:30 emoncms.log
drwxr-xr-x 2 emonhub emonhub 80 May 24 07:17 emonhub
drwxr-xr-x 2 pi root 60 May 22 08:39 emonpilcd
-rw-r----- 1 root adm 166594 May 24 07:13 kern.log
drwxr-xr-x 2 pi pi 80 May 24 07:17 logrotate
-rw-r----- 1 root adm 103685 May 24 07:13 messages
drwxr-xr-x 2 mosquitto mosquitto 60 May 22 08:39 mosquitto
-rw-rw-rw- 1 root root 0 May 22 08:39 mqtt_input.log
drwxr-xr-x 2 mysql adm 60 May 22 08:39 mysql
-rw-rw-rw- 1 root root 0 May 22 08:39 mysql.log
-rw-rw-rw- 1 root root 0 May 22 08:39 ntp_update.log
drwxr-xr-x 2 root root 40 May 22 08:39 openhab
drwxr-xr-x 2 root root 40 May 22 08:39 openhab2
drwxr-xr-x 2 redis redis 60 May 22 08:39 redis
-rw-rw-rw- 1 root root 0 May 22 08:39 service-runner.log
drwxr-xr-x 2 root root 60 May 22 08:39 supervisor
-rw-r----- 1 root adm 645955 May 24 07:17 syslog
-rw-r----- 1 root adm 1088389 May 23 13:17 syslog.1
-rw-r----- 1 root adm 1668 May 22 08:39 user.log
-rw-rw-r-- 1 root utmp 4992 May 24 06:50 wtmp
pi@emonpi:~/backup $