Perhaps I was the last person on Earth who moved from 32-bit Linux to 64-bit Linux. Finally, it has happened. The reason is simple: I needed docker (which supports 64-bit only), and it’s much nicer to create 64-bit VMs with virtualbox.
Fortunately I had a separate /home partition, so the reinstall was pretty straightforward: mess with anything, but /home. Now it’s time to reinstall a few things, eg. ansible, libreoffice, jre 8, and friends.