By "site migration" do you mean upgrading your entire 1.9.10+ site directly to 2.2.10?
Yep! That does mean all users stay, of course, none to add. All courses are there and function ... depending upon what mods/blocks, etc. used.
Different strokes for different folks ... one advantage of 'starting over' with fresh install of 2.x and then attempting to restore 1.9 backups is teachers might have to actually re-consider how the course is designed. A forward thinking teacher might now change to consider the end in mind ... ie, the student ... and what tech the students might have ... ie, iPhone/iPad, Android/Tablet, etc..
What's that "special" restore to move courses out of Legacy status?
As teacher: backup current course - full. Then, start a new course with similar title as old course, minimal config of course, when one got to the step of enrolling users, stop and go to restore, choose the backup just made, in the form that ask how to restore, restore to existing course replacing it ... the one the teacher is in.
That used to work ... it did in 2.1, and 2.2 ... did at first with 2.3 but somewhere in the updates to 2.3 and of course now in 2.4 and 2.5, no longer.
Please see:
http://docs.moodle.org/24/en/Beginning_Moodle_2.0_Administration
Why am I having trouble restoring my existing Moodle 1.9.x courses into Moodle 2.0?
Simply put, the changes in coding between Moodle 1.9.x and Moodle 2.0 are large. Additional security has forced changes to the backup processes. In just looking at the databases there are an additional 60 or more tables, so that creates a number of restoring issues alone. To bring Moodle 1.9.x courses into Moodle 2.0 is a monumental challenge, and full credit to those who are trying it. There is, currently, one commercial Windows based solution but it is not fully successful in its restoration of courses.
NOTE: Moodle 2.0 can backup and restore courses from within itself perfectly, the issue appears to be centred on v1.x.x courses.
UPDATE: Moodle 2.1 was released on July 1, 2011 and can now restore course files from backups made in v1.9.x. Student file restorations are not yet available but are expected to be included in v2.2 to be released November, 2011.
UPDATE Moodle v2.2 will restore courses and User data
UPDATE Moodle v2.4 will NOT restore courses or User Data from any v1.9.xxx or earlier. It is recommended that you install a v2.2.x and use that as a staging point.
One strategy ... in the 2, create file system repository/19backups/ and copy all the 1.9 data directories from moodledata into that repo. If needed, a file system repo can be made for a teacher and they will have access to all the files that were contained in their old 1.9 course. They just have to re-link.
Might want to clean out from those course ID folders, the backupdata directory and it's contents, moddata folder and it's contents,
'spirit of sharing', Ken