Change Usernames Of Users (serious issue)

Hi

I need to change the usernames of a group of users on my blog.
Their usernames are their email addresses.

The reason is because I transferred database information over to my site (the usernames where email addresses).

Now these poor people are getting spammed sickenly because their email adresses are all over the site.

I really need to change those usernames.
I cannot delete those accounts and start from scratch…since it’s about 70 users.

Is there a hack or something I can use? Like in phpmyadmin or something?
I will be manually changing their usernames btw.

Please help!!

Regards
Braddock