Zabbix agent upgrade from 2.x to 3.4

It's not easy as it seems. After you execute the first steps...
wget http://repo.zabbix.com/zabbix/3.4/ubuntu/pool/main/z/zabbix-release/zabbix-release_3.4-1+xenial_all.deb
dpkg -i zabbix-release_3.4-1+xenial_all.deb
apt update
apt install zabbix-agent -y
You suddenly realize that something is definitely wrong because the agent dies.

Jul 23 14:52:22 sss systemd[1]: Failed to start Zabbix Agent.
Jul 23 14:52:22 sss systemd[1]: zabbix-agent.service: Unit entered failed state.
Jul 23 14:52:22 sss systemd[1]: zabbix-agent.service: Failed with result 'exit-code'.
dpkg: error processing package zabbix-agent (--configure):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for libc-bin (2.23-0ubuntu10) ...
Processing triggers for systemd (229-4ubuntu21.2) ...
Processing triggers for ureadahead (0.100.0-19) ...
Errors were encountered while processing:
 zabbix-agent
E: Sub-process /usr/bin/dpkg returned an error code (1)

Some investigation shows /etc/zabbix/zabbix_agentd.conf.d directory does not exists and that's where the new agent looks for its configs and foolishly it does not create it. But you may have existing userparameter configs in existing /etc/zabbix/zabbix_agentd.d so the best way to continue the installation with:
ln -s /etc/zabbix/zabbix_agentd.d /etc/zabbix/zabbix_agentd.conf.d
service zabbix-agent restart
service zabbix-agent status