One’s ceiling may be more or less stable, but there are many instances where individuals have certain underdeveloped cognitive skills (of which there are a litany), undergo training to develop those skills, and then afterwards go on to score (sometimes much) higher on IQ tests. Children with certain disabilities such at autism or FASD tend to see more dramatic differences. This isn’t to say they “became more intelligent,” but rather that the testing is unable to measure intelligence directly, rather relying on those certain cognitive skills as a proxy for intelligence.