I just migrated to 18.4 a few weeks back. It is more elegant if you can back-up everything and start with a fresh install of Ubuntu rather than a update. I like to then download the entire blockchain with core. It takes a week, but then you are set for a long time.
Install Bitcoin Core
Add Bitcoin Personal Package Archive (PPA):
You will be prompted for a password. Enter the one you use to log into Ubuntu.
Next, update Ubuntu’s software package lists:
Now, install Bitcoin core itself. Enter “Y” when asked asked for permission to continue:
This command installs Bitcoin Core and all of its dependencies.