In 1998, the number of workers in category U was 8% of 2000, which equals 160 workers. In 2000, the number of workers in category U was 10% of 2400, which equals 240 workers. The percentage increase in the number of workers is calculated as follows:
\[
\text{Percentage increase} = \frac{240 - 160}{160} \times 100 = 50%
\]
Thus, the number of workers in category U increased by 50% from 1998 to 2000.