How is it possible that you can change your Windows password without re-encrypting a hard disk that was encrypted using that password?
I’ll assume you mean BitLocker whole-disk encryption, but the concept applies to many different encryption tools. You can often change the password (or passphrase) without needing to re-encrypt whatever it is you’ve encrypted.
The secret is simply this: your password wasn’t used to encrypt the disk.
BitLocker is Microsoft’s full-disk encryption technology available in Windows Pro, Enterprise or Ultimate editions from Vista onwards.
I typically recommend avoiding it, for one simple reason: it’s too easy encrypt yourself into a corner and lose access to your encrypted data.
I’ll review why I feel that way, and what steps you need to take if you want to use BitLocker safely.
For the record, I actually now use BitLocker for whole-drive encryption. Microsoft had made it harder to encrypt yourself into a corner. That doesn’t mean it can’t be done, so do follow the instructions in this article. See also: How Do I Encrypt a Disk?