First, I needed to remove the old apt repository and signing key. (This prevents do-release-upgrade -d from stopping halfway through.)

Remove repos:

cd /etc/apt/sources.list.d
sudo rm maria*

Remove old key:

# List the keys
sudo apt-key list
# Remove the correct key, which is the second half of PUB STRING1/STRING2
sudo apt-key del STRING2

Second, I needed to add the new apt repository and signing key:

# Probably is already installed
sudo apt-get install software-properties-common
# Get the key
sudo apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 0xF1656F24C74CD1D8
# Add the xenial repository
sudo add-apt-repository 'deb [arch=amd64,i386,ppc64el] http://mirrors.accretive-networks.net/mariadb/repo/10.1/ubuntu xenial main'

Now you are safe to run do-release-upgrade -d without any halts!

You’ll probably want to backup your `my.cnf` file too. The upgrade process will ask you to keep or overwrite your old one, and neither of those options are usually very desirable if you have tuned your MariaDB configuration at all.

<3


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