Index: src/wp-admin/includes/schema.php
--- src/wp-admin/includes/schema.php
+++ src/wp-admin/includes/schema.php
@@ -245,8 +245,8 @@
 
 	// Multisite global tables.
 	$ms_global_tables = "CREATE TABLE $wpdb->blogs (
-	blog_id bigint(20) NOT NULL auto_increment,
-	site_id bigint(20) NOT NULL default '0',
+	blog_id bigint(20) unsigned NOT NULL auto_increment,
+	site_id bigint(20) unsigned NOT NULL default '0',
 	domain varchar(200) NOT NULL default '',
 	path varchar(100) NOT NULL default '',
 	registered datetime NOT NULL default '0000-00-00 00:00:00',
@@ -263,7 +263,7 @@
 ) $charset_collate;
 CREATE TABLE $wpdb->blogmeta (
 	meta_id bigint(20) unsigned NOT NULL auto_increment,
-	blog_id bigint(20) NOT NULL default '0',
+	blog_id bigint(20) unsigned NOT NULL default '0',
 	meta_key varchar(255) default NULL,
 	meta_value longtext,
 	PRIMARY KEY  (meta_id),
@@ -271,24 +271,24 @@
 	KEY blog_id (blog_id)
 ) $charset_collate;
 CREATE TABLE $wpdb->registration_log (
-	ID bigint(20) NOT NULL auto_increment,
+	ID bigint(20) unsigned NOT NULL auto_increment,
 	email varchar(255) NOT NULL default '',
 	IP varchar(30) NOT NULL default '',
-	blog_id bigint(20) NOT NULL default '0',
+	blog_id bigint(20) unsigned NOT NULL default '0',
 	date_registered datetime NOT NULL default '0000-00-00 00:00:00',
 	PRIMARY KEY  (ID),
 	KEY IP (IP)
 ) $charset_collate;
 CREATE TABLE $wpdb->site (
-	id bigint(20) NOT NULL auto_increment,
+	id bigint(20) unsigned NOT NULL auto_increment,
 	domain varchar(200) NOT NULL default '',
 	path varchar(100) NOT NULL default '',
 	PRIMARY KEY  (id),
 	KEY domain (domain(140),path(51))
 ) $charset_collate;
 CREATE TABLE $wpdb->sitemeta (
-	meta_id bigint(20) NOT NULL auto_increment,
-	site_id bigint(20) NOT NULL default '0',
+	meta_id bigint(20) unsigned NOT NULL auto_increment,
+	site_id bigint(20) unsigned NOT NULL default '0',
 	meta_key varchar(255) default NULL,
 	meta_value longtext,
 	PRIMARY KEY  (meta_id),
@@ -296,7 +296,7 @@
 	KEY site_id (site_id)
 ) $charset_collate;
 CREATE TABLE $wpdb->signups (
-	signup_id bigint(20) NOT NULL auto_increment,
+	signup_id bigint(20) unsigned NOT NULL auto_increment,
 	domain varchar(200) NOT NULL default '',
 	path varchar(100) NOT NULL default '',
 	title longtext NOT NULL,
Index: src/wp-admin/includes/upgrade.php
--- src/wp-admin/includes/upgrade.php
+++ src/wp-admin/includes/upgrade.php
@@ -3711,7 +3711,7 @@
 	}
 
 	// Multisite schema upgrades.
-	if ( $wp_current_db_version < 25448 && is_multisite() && wp_should_upgrade_global_tables() ) {
+	if ( $wp_current_db_version < 60421 && is_multisite() && wp_should_upgrade_global_tables() ) {
 
 		// Upgrade versions prior to 3.7.
 		if ( $wp_current_db_version < 25179 ) {
@@ -3725,6 +3725,20 @@
 			$wpdb->query( "ALTER TABLE $wpdb->blogs CHANGE COLUMN archived archived varchar(1) NOT NULL default '0'" );
 			$wpdb->query( "ALTER TABLE $wpdb->blogs CHANGE COLUMN archived archived tinyint(2) NOT NULL default 0" );
 		}
+
+		// Upgrade versions prior to 6.9
+		if ( $wp_current_db_version < 60421 ) {
+			// Convert ID columns from signed to unsigned
+			$wpdb->query( "ALTER TABLE $wpdb->blogs MODIFY blog_id bigint(20) unsigned NOT NULL auto_increment" );
+			$wpdb->query( "ALTER TABLE $wpdb->blogs MODIFY site_id bigint(20) unsigned NOT NULL default 0" );
+			$wpdb->query( "ALTER TABLE $wpdb->blog_versions MODIFY blog_id bigint(20) unsigned NOT NULL default 0" );
+			$wpdb->query( "ALTER TABLE $wpdb->registration_log MODIFY ID bigint(20) unsigned NOT NULL auto_increment" );
+			$wpdb->query( "ALTER TABLE $wpdb->registration_log MODIFY blog_id bigint(20) unsigned NOT NULL default 0" );
+			$wpdb->query( "ALTER TABLE $wpdb->site MODIFY id bigint(20) unsigned NOT NULL auto_increment" );
+			$wpdb->query( "ALTER TABLE $wpdb->sitemeta MODIFY meta_id bigint(20) unsigned NOT NULL auto_increment" );
+			$wpdb->query( "ALTER TABLE $wpdb->sitemeta MODIFY site_id bigint(20) unsigned NOT NULL default 0" );
+			$wpdb->query( "ALTER TABLE $wpdb->signups MODIFY signup_id bigint(20) unsigned NOT NULL auto_increment" );
+		}
 	}
 
 	// Upgrade versions prior to 4.2.