There are no changes from Matthew 7:7–8.
In the context of the preceding verses, these two answer the question: “How shall we learn how to judge righteously?”
The answer is to turn to God, who will answer. The imagery is one that was well known in ancient Jerusalem, and a practice with which we are also familiar. If someone knocks on your door, you open to see who it is. Thus, if you symbolically knock on God’s door, He will open it to you.
When we communicate with God, we ask for what we need. In the context of the preceding verses, it might be to learn how to judge. If we seek that answer, we will find it. However, the imagery is not limited to the question implied by the context. It is a general comfort to know that God is anxious to help us find what we are looking for. What we must do is approach God in humility and faith.