If you just upgrade from pre-multienrol to multienrol, Moodle should
continue to work correctly -- help us double check that it works for
you.
I'm afraid it doesn't (at least not 100% cleanly). As part of the database upgrade, it says:
1146: Table 'moodle.mdl_couse' doesn't exist
Error
It seems there is a small typo (prefix_couse instead of prefix_course) at line 1661 of lib/db/mysql.php (line number corresponds to MOODLE_16_MULTIENROL branch). Ooops! I see there is the same typo in lib/db/postgres7.php (line 1403). I guess "copy & paste" is the culprit here
Saludos. Iñaki.