If it were me, I'd copy the DB over to a lab and cook up a MySQL script to automate it for me. When you're confident in your script run a few tests and then apply it to my prod DB. If it explodes then you can reload the database dump you used to setup the test (or a fresh one done just before running the script in prod, preferably).
Good sysadmins are lazy: Why do something even twice when you can automate it only once?
If that kind of scripting is not in your skillset, then I'd do it by looking up each lead, or maybe editing the list in Excel prior to uploading it (or download it, do the changes and re-upload in a fresh list). Well, that's a lie, I'd just research the scripting skills I need
but sometimes time won't allow for that :/
The important thing to remember, no matter what strategy you use, is to take great care before editing MySQL tables directly. You only do so after you've backed up everything (at least the DB, but I'd recommend the whole system using the backup scripts in the extras directory of you Vicidial install), tested thoroughly in a lab (Both the changes and the backup itself!) and advised the prod team of potential explosions if for some reason you have to do this during prod hours. The interface provides all kinds of security against human errors, whereas SQL clients won't bother to ask "are you sure" if you accidentally tell it to "Overwrite everything with NULL"