I have a Redmine instance from the package manager of Debian 8 (2.5.2.devel) and moved it to a package manager version on Ubuntu 18 (3.4.4 stable). Since it runs an Apache, Redmine should continue to use Apache as a proxy. Also, it should continue to use MySQL as backend and a SMTP relay.

Recipe:

• apt install redmine redmine-mysql libapache2-mod-passenger
• create default instance in the apt configurator
• copy SQL dump over the newly created redmine_default database
• Migrate the database schema with /usr/share/redmine# bundle exec rake db:migrate RAILS_ENV=production
• This caused various errors like "Mysql2::Error: Table 'email_addresses' already exists". I had to rename the following tables and remove an index that interfered with the migration: custom_field_enumerations, import_items, email_addresses, imports, roles_managed_role. All tables were empty except the email_addresses. However, the data didn't seem to be necessary anywhere.
• Copy all files from and to /var/lib/redmine/default/files, chown to www-data:www-data
• Copy custom SMTP config from and to /etc/redmine/default/configuration.yml
• Add PassengerDefaultUser www-data to /etc/apache2/mods-enabled/passenger.conf
• Copy and adjust vhost config:

<VirtualHost *:80>
   ServerName x

   RailsEnv production
   SetEnv X_DEBIAN_SITEID "default"
   SetEnv RAILS_RELATIVE_URL_ROOT "/"

   PassengerAppGroupName redmine_default

   Alias "/plugin_assets/" /var/cache/redmine/default/plugin_assets/
   Alias "/" /usr/share/redmine/public
   <Location />
        PassengerBaseURI /redmine
        PassengerAppRoot /usr/share/redmine
   </Location>
   <Directory "/usr/share/redmine/public">
        Allow from all
        Options -MultiViews
        Require all granted
   </Directory>
</VirtualHost>