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

src/components/structures/MatrixChat.js

@@ -1033,6 +1033,7 @@ module.exports = React.createClass({
                     idSid={this.state.register_id_sid}
                     email={this.props.startingQueryParams.email}
                     username={this.state.upgradeUsername}
+                    disableUsernameChanges={Boolean(this.state.upgradeUsername)}
                     guestAccessToken={this.state.guestAccessToken}
                     defaultHsUrl={this.props.config.default_hs_url}
                     defaultIsUrl={this.props.config.default_is_url}

src/components/structures/ScrollPanel.js

@@ -19,7 +19,7 @@ var ReactDOM = require("react-dom");
 var GeminiScrollbar = require('react-gemini-scrollbar');
 var q = require("q");
 
-var DEBUG_SCROLL = false;
+var DEBUG_SCROLL = true;
 
 if (DEBUG_SCROLL) {
     // using bind means that we get to keep useful line numbers in the console

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} />