Hello everyone, this is not official ! (if Support want to edit it for validation, do it !)
This is my upgrade process that works on my side ( Debian 9 Stretch ).
It is based on upgrade V2 -> V3.
My server was up to date, all packages updated, latest iredmail and iredadmin installed.
No errors on apt update. Running good.

First we make a Backup !

mkdir -p /root/sogo-backup/{sogo,sysconfig}
cp /etc/sogo/* /root/sogo-backup/sogo/
cp /etc/sysconfig/sogo /root/sogo-backup/sysconfig/

Next we remove Sogo configuration :

apt-get remove "libsope*" "sogo*"

Next we update the source list and replace "4" by "5" :

nano /etc/apt/sources.list.d/sogo-nightly.list
deb https://packages.inverse.ca/SOGo/nightly/5/debian/ stretch stretch

Now we install Logo in Version 5 ! :

apt-get install sogo

Answer : Yes package maintainer.

Now we can restore our configuration file and update permissions on log :

cp /root/sogo-backup/sogo/* /etc/sogo/
cp /root/sogo-backup/sysconfig/sogo /etc/sysconfig/
chown sogo:sogo /var/log/sogo/sogo.log

We finished by restarting the service.

service sogo restart