Ahhh sooooo .... inherited sites with no docs/notes left by former admins is always an adventure and will test ones troubleshooting abilities + skills in areas of DB and maybe even coding.
So you do not have a stock Moodle, but one that might have been highly customized. Ugh!
* leave debugging on!!!! *
Have been through the process of converting users authentication from manual to LDAP en mass and it's a semi-tricky process.
First, use the Moodle UI as admin to export the users out into a CSV file so you can inspect what data is contained in mdl_user.
Site Admin -> Users -> Bulk User Actions -> Add All
Then use With Selected Users drop down menu select Download - click Go
You will get a screen displaying file format ... Txt, ODS, Excel. I'd select Txt (you might end up using Excel anyway, but ... Excel does have some quirks sometimes.
That will download a file entitled users-[datetimestamp]-comma-separated.csv
User ID #1 should be guest and you may/maynot see it (member, your site has been customized and one doesn't know how much). User ID #2 was/is the original installers account ... normally, if left for defaults when installed, that username was admin. Former admin could have changed that, however.
First line in the CSV are the fields from the mdl_user table and must be present when you bulk import. But, one doesn't need to include all the fields ... just the ones you desire to change (add).
IT will look something like this:
4,hpotter,hpotter@someaddy,Harry,Potter,,,,,,"Hogwarts, The Cloud",,,,,,,US
Note there is no password field in the export and there is no authentication field.
* remove the account you are using to access the Moodle ... that one should be using manual authentication ... and any other users that are admin levels. Those accounts do authenticate vs the mdl_user table.
Then one has to doctor the csv file (can do with spreadsheet).
You do NOT need the ID column. You do need a minimium of username,email, firstname,lastname. AND, you will insert two columns ... one for auth and **you might need** one for password that will be set the same for all user accounts you will be changing to 'changeme'. (external authentications don't copy the password back to Moodle DB normally)
** Does your entity have LDAP? Is that an option for this Moodle instance? Then you don't need password field when importing for Moodle will use LDAP to check password of user logging on ... and Moodle won't copy the password to it's table.
If so, then the 'auth' column will say 'ldap' ... but this is where it's tricky ... you have to know how LDAP is setup. That changes things ... by that I mean, the ID of students in LDAP might be their student ID number ... which could be used as their 'username' in systems you are interfacing.
At this point cannot give any more specific details as much depends upon setup systems, etc..... but think I've given enough to get you started.
This situation is not fun and it's good you are doing this on test system first.
You can turn off performance thang to display on pages ... that has nothing to do with things here and really slows down things even more. Also anything else you may have turned on (cept debugging) OFF to investigate/solve this issue.
'spirit of sharing', Ken