I'll use password to refer to what you know, and key to the random key used to actually encrypt the data.
A password has a lot of exposure. No matter how strong it is, someone might:
- use a keylogger
- watch over your shoulder as you type
- it might be captured on a "security" camera
- you might be coerced into giving it up
These are all reasons to change the password, even if the key is left unchanged.
An attacker can try to crack the key directly, but a key is generally something like 128 or 256 bits of random data. In order to brute force that, you'd use an amount of energy somewhere between boiling the earth's oceans and all of the Sun's output. So that's not really a risk at all.
The only reason to change the key, is when someone obtained the password and managed to decrypt the key before you could change the password. At that point, they know the key and changing the password without the changing the key is not going to help anymore.
By the way, good question! It seems like a simple question with a simple answer, but by reasoning about it in this answer, I actually learned a few things. Like that changing the key might be necessary after a password compromised. I hadn't given that thought before and assumed that the data encryption key could always be static.