Steps to reproduce:

1. Install the agent package on Debian 9 via the link

2. wget http://repo.zabbix.com/zabbix/3.4/debian/pool/main/z/zabbix-release/zabbix-release_3.4-1+stretch_all.debdpkg -i zabbix-release_3.4-1+stretch_all.deb
   apt-get update

3. Make changes to the /etc/zabbix/zabbix_agentd.con file
4. Wait for a agent upgrade via apt-get -y install zabbix-agent

Result:
The /etc/zabbix/zabbix_agentd.conf file is renamed to zabbix_agentd.conf.dpkg-old
A new default zabbix_agentd.conf file is place in /etc/zabbix

Expected:
A package upgrade should not replace existing config files, but instead place the new config files as zabbix_agentd.conf.dpkg-new