I got a message that Aperture needs to upgrade my library, once upgraded, I will not be able to use this library.
Photogirl, that surely is only half of the alert message. You will not lose your photos; this message is alerting you to the fact, that there will be no way back. Once the library has been upgraded, you will no longer be able to open it in the previous Aperture version. So, if you have other macs with older Aperture versions, these will no longer be able to read your converted Aperture library.
When you see an alert like this, it is always wise to check, if your backup is current, before you proceed. Make sure, you have a working backup of your Aperture library, and also, of your current version of Aperture, just in case.