@elberet, thanks for your help so far. I went for the second option - connecting from the Discourse machine to the MySQL server of the running forum (set to maintenance mode) after enabling port 3306 in the firewall and setting bind-address = 0.0.0.0 in /etc/mysql/my.cnf followed by a sudo service mysql restart. Here’s what I’ve done based on your advice:
cd /var/discourse
./launcher enter app
apt-get update
apt-get install libmysqlclient-dev mc -y
gem install mysql2 # mysql2-0.3.18 got installed
su - discourse
cd /var/www/discourse/script/import_scripts
bundle install
mcedit mybb.rb # make edits to the MySQL connection details
RAILS_ENV=production ruby mybb.rb # no idea what "bundle exec" does
To setup redirects from the old forum thread URLs to Discourse topics, see my other post:
https://meta.discourse.org/t/redirecting-old-forum-urls-to-new-discourse-urls/20930/21?u=dandv