What version of PHP are you running the 1.9 Moodle site on?
Bear in mind that you will need to upgrade your theme during the upgrade march to 3.6. Your 1.9 theme won't work in 3.6.
And you can't upgrade from 1.9 directly to 3.6. You have to do it in stages. I would use a strategy similar to this:
- Upgrade 1.9 to 2.0 if you have a LOT of quiz attempts that you want to migrate. Otherwise you can go from 1.9 to 2.1. Moodle version 2.0 introduced a new Question Engine, so you'll need to check that all your quiz attempts upgraded successfully .
- Make sure you're on PHP 5.3.2 then upgrade 2.0 (or 2.1) to 2.2.11.
- Upgrade PHP to version 5.4.
- Upgrade Moodle 2.2.11 to 2.7.20.
- Upgrade Moodle2.7.20 to 3.1.14.
- Upgrade PHP to version 7.0.31.
- Upgrade Moodle 3.1.14 to the latest 3.6 version.
- Upgrade PHP to the latest version.
The differences between Moodle 1.9 and Moodle 3.6 are huge. You'll need to do some kind of student orientation when you introduce them to 3.6 to make the transition easier for them.
Importantly, before you attempt the uprade read the Moodle documentation for each version carefully. Each document will explain the requirements for that version, eg.:
And read the Release Notes for each version on https://docs.moodle.org/dev/Releases. Doing that will give you an indication of what the changes are for each version.
If you have any non-core plugins in your 1.9 site you'll need to check if there are compatible versions for each of the Moodle versions above.
Lastly before upgrading your production server you should do a full backup of the production server and restore that backup onto a test server. Then test the upgrade on the test server and document the process fully. You should do thorough testing on the test server after each Moodle and PHP version upgrade.
So, as you can see, this upgrade is a massive undertaking. You need to decide if it's worth it or if it's easier to just create your courses again in a blank new Moodle 3.6 site.
Lastly, your Moodle 1.9 course backup files won't work in Moodle 3.6. The backup format changed completely in one of the latter 2.x versions. That means you can't restore Moodle 1.9 backup files into a new Moodle 3.6 site.
Browse these forums to see how others have done the upgrade and the issues that they had. Example: https://moodle.org/mod/forum/discuss.php?d=312810