We plan to upgrade and interested to know what is the major differences between 2.9 and 3.3?
How much would it affect students jumping from 2.9 to 3.3?
Going from 2.9 to 3.3 (though 3.4 is the latest version!), you may require some changes to your theme or plugins (if using third party) and may also require a change in how you run your courses.
The biggest change that may cause confusion for users I think will be the new menu system, but the old one can be returned if you require.
The best way to find how it would affect your students is to test Moodle 3.3 yourself and look at how different things are for your site.
You should consider moving to Moodle 3.4 if your server can accommodate it.
If I were you, I would set up Moodle 3.4 as an experimental on an exactly configured server, then work with it for at least a good month or two. Then you could be ready to install a "production" moodle.