I ran some tests on a new wallet:

1) No password
http://pastebin.com/MZ6FHcNV

2) Added a password
http://pastebin.com/CMzAw4bF

3) Changed the password (was not allowed by the GUI or debug console to remove it)
http://pastebin.com/kJQzykNW

Basically, adding a password changes the xprv, but changing it does not.
Also, all the keys from the pool are added to the new wallet file after changing the password.

So basically, if you change the password, in order to restore you need to either:

- Keep the wallet file
- Only keep the xprv, as long as you are sure you are not going to receive to the addresses derived from the previous xprv

If I'm wrong, please correct me.