There are no changes from Matthew 7:9–11.
These verses are the expansion of the previous verses admonishing us to ask God for what we need. The reason for these statements is to help the hearers understand the nature of God. God is powerful, and therefore it would be understandable to approach him with fear and trembling. We might not know how we will be received.
In the New World, this was particularly apt, for it was in the beginning of that same year that the earth had undergone tremendous destruction, and there had been thick darkness for three days. For those at Bountiful, it was abundantly clear that God had power, and that power could be destructive and fearful.
Therefore, it was important to emphasize that, as a good earthly father, our Heavenly Father knows how to give good gifts.
Verses 9 and 10 are images that are more appropriate to Jerusalem than to the New World. The typical bread that was baked was not in formed pans as we know them, but closer to a half sphere. Therefore, their bread and a stone had at least a visual similarity.
The common fish in Galilee was long and thin, and had therefore a kind of visual similarity to a serpent. Both images are suggesting that if we ask for bread, or a fish, a kind father would not give us just something similar, and which did not meet our needs.