With 18.04 LTR coming soon, it's probably time to start working on migrating my production machines from 14.04 to 16.04. I have a new 16.04 testing system running. What is the recommended procedure for migrating Kontact data and settings, especially email folders, from 14.04 to 16.04?

You will want to use this https://docs.kde.org/stable5/en/pim/pim ... index.html

Out of interest, how come you won't go to straight to 18.04?