by Mark Jones.
We just converted about 700 courses from our 1.9 environment. i would suggest that you set up a middle server with Moodle 2.1.4 installed. Backup and save the course from your 1.9 environment, restore the course to the 2.1.4 Moodle server, backup and save the course, now restore on your new Moodle 2.3. This sounds like a lot of work but in the long run the process works very well.
Also here are the setting i use for the really large courses to convert:
change this in the php.cfg file
max_execution_time =19200
max_input_time = 1500
max_input_vars = 8000 memory_limit = 1024M
post_max_size = 1200M
upload_max_filesize = 1000M max_file_uploads = 30
FastCGI Settings
Activity Timeout 3600
idle Timeout 3600
Queue Length 2000
Request Timeout 3600