And already given somewhere else, but I don't remember.
Here is my overcme (not the best, but working!)
-Insatll (new install no upgrade) 1.9.x
-from database, save (export using sql format) the 'user' table from the 1.8
-from database, table 'user' for 1.9.x, remove the 2 accounts (admin and John Doe)
- from database, import 'user' table you just saved in 1.9
At this point you have a brand new 1.9 version tha has ALL the user you had, with their password!
Now you have to export each course (from Maintenance in the course) from 1.8 and import them, then register your trainees.
I know it's not as easy as upgrading the 1.8, but it's efficient