I would like to change my pgp key because I have the same one 2 years ago. But, I want to warn my freinds that the owner of the old key and the owner of the new one is the same person (me). hwat is the way I can use to prove I am the owner of the new pgp key ? Include it on the revocation certificate of the new one ?
The main thing you need to do is revoke any usage of your old certificate. To do this you probably want to both upload your revokation certificate to any key servers you used to share your public key.
In addition it wouldn't go amiss to send this to all your contacts you use PGP with, so that they can manually revoke/remove your old key.
There is no way to prove to your contacts using the revocation certificate that the new public key is yours. This is to be expected and you really should be aware that the very ability to issue a recovcation certificate indicates access to the private key, something an attacker could just as well have (then using social engineering, convince users like you're trying to do to use their public key).
You need to negotiate distributing your new key manually, by other secure means. The most convenient way is probably uploading your new public key to a key server and then calling your friends with the fingerprint ID to confirm they pick the right one.
I would personally probably sign my new public key with the old private key and send it to my friends with an e-mail explaining the change. They could then verify I was the holder of the previous key and are made aware of the change. I'd then post a revocation of the old certificate and include the details of the new one if supported. (I'm not that particularly familiar with PGP, just general PKI.)