Yubico offers free personalization tools for Windows, Mac and Linux allowing you to re-initialize the AES key in your YubiKey.
If you choose to re-initialize the AES key of your YubiKey, Yubico will not be aware of the new AES key and can't validate the One-Time Passcode (OTP) generated from it. To validate OTPs generated from your newly programmed YubiKey, you need to host your own YubiKey validation server and put the same AES key in the server's database.