Follow up .... just to make certain you understand about the excel work for creating a users csv update file.
Remove the rows in excel that show 'manual' as authentication - those would be guest and the initial account created for admin access - typically, user ID 1 and 2. And, if you had created another admin level account to use ... it's also set to manual.
You only want to update the student/teacher accounts that had used the old googleoauth2 for authentication.
Passwords were not retained in mdl_user table using the old and won't be in the new. So you need not worry about them.
And for the how to for uploading users:
https://docs.moodle.org/33/en/Upload_users
Got any students with names that use 'a-typical' characters? Knew a guy one time whose first name was "D'Jack" ... this to remind to check over the saved .csv file from excel with notepad to assure you select the correct delimiters in the CSV file and that a name like 'D'Jack' doesn't bite ya! ;)
Ok, think I'm done! ;)
'spriit of sharing', Ken