What does Bitcoin-Qt "Change Passphrase..." do to existing private keys and corresponding addresses? "Change Passphrase..." obviously lets me encrypt the wallet with a new string of text.  I think (correct me if I'm wrong) that this creates new private key(s) inside wallet.dat.  

How are existing addresses affected when the passphrase is changed?  


Are they still linked to the old private key (previous passphrase)?  
If so, should I send those Bitcoin to myself (sending them to a new address that was generated after the change of passphrase)?
The wallet actually uses a randomly generated master key to encrypt the keys, and this master key is encrypted using your passphrase.

When you change the passphrase, all that changes is the encrypted master key. The actually encrypted wallet keys aren't touched at all.