User Name and User Email Conflict?
I am puzzled by your description of the problem.  Drupal 7 should not complain if  A user name is the same as account B email address.

However, without making a too big deal of that, there is a way you can change an email address to avoid any conflict within Drupal, and still get mail delivered to that user without interruption.

Simply insert the character + and some string after the name part of the address, like this:

User account A:
Username   - usernameX
user email - usernameX+a@gmail.com

User account B:
username   - usernameX@gmail.com
user email - usernameB@gmail.com


Drupal allows email-adresses with a + in them.  

And for string comparison purposes:

usernameX+a@gmail.com ≠ usernameX@gmail.com


While for mail delivery purposes:

usernameX+a@gmail.com = usernameX@gmail.com


This technique, known “plus-addressing” allows users to sign up for different services with different aliases and then easily filter all e-mails from those services (provided their mail server doesn't strip it). Mail servers deals differently with “plus-addresses”. MS Exchange strips it, while Cyrus and GMail retains it - but AFAIK all mail servers deliver mail with “plus-addresses” to the intended recipient.