core-bundle/src/Resources/contao/library/Contao/Database/Updater.php
public function run35Update()
	{
		$this->Database->query("ALTER TABLE `tl_member` CHANGE `username` `username` varchar(64) COLLATE utf8_bin NULL");
		$this->Database->query("UPDATE `tl_member` SET username=NULL WHERE username=''");
		$this->Database->query("ALTER TABLE `tl_member` DROP INDEX `username`, ADD UNIQUE KEY `username` (`username`)");
	}
public function run35Update()
	{
		$this->Database->query("ALTER TABLE `tl_member` CHANGE `username` `username` varchar(64) COLLATE utf8_bin NULL");
		$this->Database->query("UPDATE `tl_member` SET username=NULL WHERE username=''");
		$this->Database->query("ALTER TABLE `tl_member` DROP INDEX `username`, ADD UNIQUE KEY `username` (`username`)");
	}
Version 3.5.0 update