CSV Import works just perfect.
You can export a list of all the current manual users ( https://docs.moodle.org/24/en/Import_and_export_FAQ#How_do_I_export_a_list_of_users.3F ) and then tweak the exported file to what you need updated. I had to add the 'auth' and 'password' columns.
Here is what my test CSV looked like. The second account was an admin account and Moodle actually stopped it from getting updated as it was an admin account. Great feature to cover your butt just in case.
username,email,firstname,lastname,auth,password
user1,user1@domain.ca,manual,user1,ldap,
shawnp,shawnp@domain.ca,Shawn,P,ldap,