Suppose both the receiver and sender use PGP. Suppose someone send from the sender an email and sent it to the receiver. Now he gets the content of the email at the receiver. Can he know now the keys and decrypt the past and future emails. Can he extract from this process the keys?
Your question is not very clear, but in public-key cryptography, everyone has a public key which can be freely distributed, and a secret key which only that person knows.
To send a secret message to someone, you encrypt it with their public key. It can only be decrypted with the secret key.
To sign a message, you use your secret key. No one else can sign it with that key, but anyone can verify the signature using your public key.
Does that clear things up?
It does not matter past or future as long as you have public/private keys you can decrypt any encrypted messages with those keys.
What I read from your comments,which is your case, if you start using NEW private key that you start encryting messages with then in your case user cant read your messages with old private key.Because you have NEW pairs.