diff --git a/wp-admin/includes/class-wp-upgrader.php b/wp-admin/includes/class-wp-upgrader.php
index f2db9ef97..406032037 100644
--- a/wp-admin/includes/class-wp-upgrader.php
+++ b/wp-admin/includes/class-wp-upgrader.php
@@ -953,13 +953,11 @@ class Theme_Upgrader extends WP_Upgrader {
 		if ( $theme != get_stylesheet() ) // If not current
 			return $return;
 
-		// Ensure stylesheet name hasnt changed after the upgrade:
-		// @TODO: Note, This doesn't handle the Template changing, or the Template name changing.
+		// Ensure stylesheet name hasn't changed after the upgrade:
 		if ( $theme == get_stylesheet() && $theme != $this->result['destination_name'] ) {
-			$theme_info = $this->theme_info();
+			wp_clean_themes_cache();
 			$stylesheet = $this->result['destination_name'];
-			$template = $theme_info->get_template();
-			switch_theme($template, $stylesheet, true);
+			switch_theme( $stylesheet );
 		}
 
 		//Time to remove maintenance mode
diff --git a/wp-admin/themes.php b/wp-admin/themes.php
index 465626d2d..5a78fe3dd 100644
--- a/wp-admin/themes.php
+++ b/wp-admin/themes.php
@@ -20,7 +20,7 @@ if ( current_user_can( 'switch_themes' ) && isset($_GET['action'] ) ) {
 		$theme = wp_get_theme( $_GET['stylesheet'] );
 		if ( ! $theme->exists() || ! $theme->is_allowed() )
 			wp_die( __( 'Cheatin’ uh?' ) );
-		switch_theme($_GET['template'], $_GET['stylesheet']);
+		switch_theme( $theme->get_stylesheet() );
 		wp_redirect( admin_url('themes.php?activated=true') );
 		exit;
 	} elseif ( 'delete' == $_GET['action'] ) {
diff --git a/wp-includes/class-wp-customize-manager.php b/wp-includes/class-wp-customize-manager.php
index c1ce0510e..b561fb24e 100644
--- a/wp-includes/class-wp-customize-manager.php
+++ b/wp-includes/class-wp-customize-manager.php
@@ -475,7 +475,7 @@ final class WP_Customize_Manager {
 			// Temporarily stop previewing the theme to allow switch_themes()
 			// to operate properly.
 			$this->stop_previewing_theme();
-			switch_theme( $this->get_template(), $this->get_stylesheet() );
+			switch_theme( $this->get_stylesheet() );
 			$this->start_previewing_theme();
 		}
 
diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index ba968bca8..a99f58c40 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -650,15 +650,17 @@ function preview_theme_ob_filter_callback( $matches ) {
 }
 
 /**
- * Switches current theme to new template and stylesheet names.
+ * Switches the theme.
+ *
+ * Accepts one argument: $stylesheet of the theme. It also accepts an additional function signature
+ * of two arguments: $template then $stylesheet. This is for backwards compatibility.
  *
  * @since 2.5.0
  * @uses do_action() Calls 'switch_theme' action, passing the new theme.
  *
- * @param string $template Template name
- * @param string $stylesheet Stylesheet name.
+ * @param string $stylesheet Stylesheet name
  */
-function switch_theme( $template, $stylesheet ) {
+function switch_theme( $stylesheet ) {
 	global $wp_theme_directories, $sidebars_widgets;
 
 	if ( is_array( $sidebars_widgets ) )
@@ -666,7 +668,13 @@ function switch_theme( $template, $stylesheet ) {
 
 	$old_theme  = wp_get_theme();
 	$new_theme = wp_get_theme( $stylesheet );
-	$new_name  = $new_theme->get('Name');
+
+	if ( func_num_args() > 1 ) {
+		$template = $stylesheet;
+		$stylesheet = func_get_arg( 1 );
+	} else {
+		$template = $new_theme->get_template();
+	}
 
 	update_option( 'template', $template );
 	update_option( 'stylesheet', $stylesheet );
@@ -676,6 +684,8 @@ function switch_theme( $template, $stylesheet ) {
 		update_option( 'stylesheet_root', get_raw_theme_root( $stylesheet, true ) );
 	}
 
+	$new_name  = $new_theme->get('Name');
+
 	update_option( 'current_theme', $new_name );
 
 	if ( is_admin() && false === get_option( 'theme_mods_' . $stylesheet ) ) {
@@ -706,17 +716,17 @@ function validate_current_theme() {
 		return true;
 
 	if ( get_template() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/index.php') ) {
-		switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
+		switch_theme( WP_DEFAULT_THEME );
 		return false;
 	}
 
 	if ( get_stylesheet() != WP_DEFAULT_THEME && !file_exists(get_template_directory() . '/style.css') ) {
-		switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
+		switch_theme( WP_DEFAULT_THEME );
 		return false;
 	}
 
 	if ( is_child_theme() && ! file_exists( get_stylesheet_directory() . '/style.css' ) ) {
-		switch_theme( WP_DEFAULT_THEME, WP_DEFAULT_THEME );
+		switch_theme( WP_DEFAULT_THEME );
 		return false;
 	}