Hi,
this might seem a very simple question but I can't find a straight answer to it.
Imagine someone has access to an old backup of my wallet (QT client, Multibit, Electrum, etc...) and that this person has also the original password of that wallet.
That password has changed now, but can someone move my BTC with the old data?
Yes. Changing your password on the new wallet doesn't change it on the backup. Secure your Bitcoins by moving them to a new address which only you control immediately in such a case.