Let's assume that Bob, at some point in time, has created a PGP (key length and other parameters are not relevant at this point). For some reason, he has chosen a very poor passphrase, for instance
lemon. Some time later, Bob realises his poor choice and decides to change his passphrase to something much more secure, for instance
There is no doubt that a longer passphrase is more secure than a single word.
There's the catch: At some point before the passphrase was changed, the private key somehow leaked (e.g. due to a not appropriately secured backup scheme or some other mishap). Either way, Eve now has Bob's private key, but not its passphrase. Given the poor choice of the original passphrase, this will just be a matter of time, so for the sake of the discussion let's assume that Eve eventually manages to deduce the passphrase in some way. She thus has both the private key and its passphrase and will be able to decrypt it.
The key itself of course is compromised at this point, this is without question. Eve is certainly able to decrypt her (barely protected) copy of Bob's private key, even if Bob's copy is now encrypted in a different manner (read: new passphrase). So I guess this corresponds to a known plaintext attack on the private key. Will Eve be able to deduce Bob's /new/ passphrase (and thus possibly other keys where he used the same passphrase) in this scenario?