Here is the original study:
I believe its conclusions have been rather mangled along the way, until you have reached a false conclusion.
Does one's personality set in at seven year's old?
The study does not say that. The study says that there are a larger number of correlations between teacher ratings of students in elementary school and scientist ratings of behaviour 40 years later than one might expect through chance alone.
For example, from the abstract:
Early ‘‘adaptability’’ was associated with
cheerful and intellectually curious behavior, early ‘‘impulsivity’’ was associated with later talkativeness and loud speech, and early rated
tendencies to ‘‘self-minimize’’ were related to adult expressions of insecurity and humility.
I understand that the study says that personality can change, but it's hard.
The study does not say that. The LiveScience article attributes that to "previous research", linking to another page that gets closer to this claim, but still doesn't quite seem to make it.
Is this a large enough sample size?
Yes, it was a large enough sample size to demonstrate the effects that they claimed, which were very mild. This was demonstrated by a statistical analysis that showed the number of correlations was considerably higher than expected by chance alone.
Were there any children whose personalities drastically changed?
This contains a fundamental misunderstanding of the research.
Firstly, practically every child has a drastically different personality 40 years later. They are more mature in numerous ways. If you found a 47-year-old with the personality of a 7-year-old, they would likely be in a mental institution.
Secondly, they didn't measure the same things - e.g. "impulsivity" was measured as a child, and "loud speech" was measured as an adult, and they were found to correlate. This study could not show that personalities were the same even if they were.
Thirdly, even the strongest correlations were fairly mild. e.g. one of the strongest correlations was between "Verbal Fluency" as a child and "Shows interest in intellectual/cognitive matters" as an adult, and it was r=0.30. Without giving a full lecture on how correlation works, an r-value of 0.30 indicates a mild tendency, not a strong prediction.
Are there any other studies that conclude differently?
I considered editing this question out, because it shows a strong bias. You should keep an open mind about which direction the evidence may take you.
In summary, the paper which triggered your concerns does not make the claims that you are concerned about.