oops, revert - do this on the right branch...

src/components/structures/login/Registration.js

@@ -43,6 +43,7 @@ module.exports = React.createClass({
         email: React.PropTypes.string,
         username: React.PropTypes.string,
         guestAccessToken: React.PropTypes.string,
+        disableUsernameChanges: React.PropTypes.bool,
         // registration shouldn't know or care how login is done.
         onLoginClick: React.PropTypes.func.isRequired
     },
@@ -107,12 +108,6 @@ module.exports = React.createClass({
             errorText: "",
             busy: true
         });
-
-        if (formVals.username !== this.props.username) {
-            // don't try to upgrade if we changed our username
-            this.registerLogic.setGuestAccessToken(null);
-        }
-
         this.onProcessingRegistration(this.registerLogic.register(formVals));
     },
 
@@ -210,6 +205,7 @@ module.exports = React.createClass({
                         showEmail={true}
                         defaultUsername={this.props.username}
                         defaultEmail={this.props.email}
+                        disableUsernameChanges={this.props.disableUsernameChanges}
                         minPasswordLength={MIN_PASSWORD_LENGTH}
                         onError={this.onFormValidationFailed}
                         onRegisterClick={this.onFormSubmit} />