You can apparently order a new one sent to your address without sending the old one back for far less money. Do that instead.
They don't do that anymore either. According to them without mailing it back your pretty much screwed now. Definitely do not want to lose your yubikey or your in big troubles.
Hello,
You may still order a new Yubikey from
https://yubikey.mtgox.com/ which will replace the old Yubikey linked to your account.