Enrolment

OK.  Since our institution is in the process of implementing a new identity management system, I'm dealing with bulk account imports.  I've got an import script that's running which dumps all students who have added a class to a file for uploading.  This file includes First, Last, Email (many students don't have one), password (a number the students can lookup), username (a number the student knows) and "course1."  Since many students enroll in multiple courses, I have to use "update existing accounts" because moodle doesn't look to see if the "course1" provided is new data (I should add that this seems highly counter-intuitive to me; I HOPE there's a legitimate reason why this is the DEFAULT behaviour).

Since our school is over 50,000 students, doing this by hand is not an acceptable proposition.  As far as I can tell, I should upload this via the Upload Users screen, but I have a question (we're live; I don't wanna screw anyone up).

If a student has reset their password, and I upload with "update existing accounts" set to "yes," will the system update ALL fields, including the students' passwords?  Also, does moodle REPLACE the contents of "course1" if it's new?  In other words, if the student was uploaded once and "course1" was set to CIS101, then they added CIS102 and got uploaded again, will they be unenrolled from CIS101?

TIA!