I've a desperate need to change mine and a few of my colleagues' sign-in names in phpMyAdmin -- for professional purposes (i.e. our sign-in names would change to our full first and last names, so that when our author archives are accessed it'll be through http://site.com/author/josephgerald).
Will changing the user name in phpMyAdmin have any effect on the posts our names are already tied to?
You don't have to change your login user names. Simply add your first & last names to your profiles and then set your display name to show these - again in your profiles.
I don't think you understand.
Sure, I can change my display name to whatever I desire whenever I like, but that will never change the fact that my author URL will remain something like: http://site.com/author/homedawg4.
Will changing the user name in phpMyAdmin have any effect on the posts our names are already tied to?
I don't believe it will have any problems. Since the comments and posts are grabbed from that table, they should magically update.