Adding 2 cents ... or a little more detail ... not meaning to scare ... and hope this is still correct (am certain others will correct me if I'm wrong (please do!).
We know the goal is to get 1.9 courses with user data to a version of 2 - 2.8,2.9 but there are some things that make this a migration a little more complicated than it sounds (or have been suggested at this point).
The only way to bring users/their work/grades to a 2 is to do a site migration. What no one has mentioned yet, that while that's possible, the migration affects files/file system. Site migration of a 1.9.highest site to a 2 will bring with it something called 'Legacy File System' and the site + all courses migrated wiil be in legacy file system mod. Cannot turn that off. (note: new courses under the 2 will not use the Legacy File system). It's a trade off really ... so if users are important, live with Legacy files.
Begin this migration process by working with a copy of your existing 1.9 site ... copy of the code, copy of the DB, copy of the moodledata directory. *MASSIVE* changes will occur to the DB and to the moodledata directory.
Your 1.9 copy should be upgraded to the highest version in that series. (even Moodle.org docs say/warn that all previous updates should be applied prior to upgrading).
Before making the first step ... 1.9 -> 2.x, check the 1.9 for any addons ... mods/blocks/themes/quiz question types/themes etc. Some of those didn't have a 2.x version ready when 2.0 was released and some have not updated their code to be compat with 2.x. If it's a stock 1.9 (ie, no addons) then no worries in that department. If you have addons, research which ones have 2 compat versions ... found by searching plugins. If there is no 2.x addon, consider removing the addon in the 1.9 before attempting migration.
Yes, the path is 1.9.highest to 2.2.highest ... but, I'd advise then taking the 2.2.highest to at least 2.3 for there were some changes to version 2 of the most used activities: assignments and quiz. In the 2.3 there appeared some links from the Site Admin menu for "Assignment Upgrade Helper" and one for Quizzes as well. In 2, the assignment module changed to an assign mod. While site is a 2.3, use those tools. Convert your assignments and quiz engine.
One stands a better chance then of making a full backup of courses (including users, their assignments, test, and grades) that will restore to higher versions of 2.
'spirit of sharing', Ken