My notes, things I have learned, and documentation I have created.

I finally upgraded Yourls in an attempt to get back to the Twitter plugin. Simple to upgrade.

Backup yourls directory location.
Backup yourls MySQL database.
Remove everything from your yourls location.
Unzip the new yourls software.
Copy the config_sample.php to config.php. The configuration files are location in the includes directory.
Modify the config.php with your site specific settings.
I had to modify the following definitions:
YOURLS_DB_USE
YOURLS_DB_PASS
YOURLS_DB_NAME
YOURLS_SITE
$yourls_user_passwords

And, because I was upgrading from 1.2, I had to add the following definitions:
define(‘YOURLS_DB_TABLE_URL’, ‘url’);
define(‘YOURLS_DB_TABLE_NEXTDEC’, ‘next_id’);

Go to your yourls site:
http://yourls-url/admin
Login and upgrade.

I ended up having to just restore the old files. Someday when I get more time, I will figure out what is wrong. I believe the plugin is too new for the version of Yourls I am running (1.2).

I am having trouble with the yourls plugin for WordPress, since I updated it this morning. It keeps saying the username or password is incorrect despite manually verifying them. Might have to test sending some tweets. Sorry.