bp_pre_schema_upgrade()

function bp_pre_schema_upgrade() {
	global $wpdb;

	$raw_db_version = (int) bp_get_db_version_raw();
	$bp_prefix      = bp_core_get_table_prefix();

	// 2.3.0: Change index lengths to account for utf8mb4.
	if ( $raw_db_version < 9695 ) {
		// Map table_name => columns.
		$tables = array(
			$bp_prefix . 'bp_activity_meta'       => array( 'meta_key' ),
			$bp_prefix . 'bp_groups_groupmeta'    => array( 'meta_key' ),
			$bp_prefix . 'bp_messages_meta'       => array( 'meta_key' ),
			$bp_prefix . 'bp_notifications_meta'  => array( 'meta_key' ),
			$bp_prefix . 'bp_user_blogs_blogmeta' => array( 'meta_key' ),
			$bp_prefix . 'bp_xprofile_meta'       => array( 'meta_key' ),
		);

		foreach ( $tables as $table_name => $indexes ) {
			foreach ( $indexes as $index ) {
				if ( $wpdb->query( $wpdb->prepare( "SHOW TABLES LIKE %s", bp_esc_like( $table_name ) ) ) ) {
					$wpdb->query( "ALTER TABLE {$table_name} DROP INDEX {$index}" );
				}
			}
		}
	}
}