A cohort study in Delhi investigates the association between occupational dust exposure and chronic obstructive pulmonary disease (COPD) in factory workers. Researchers follow 500 exposed and 500 unexposed workers for 5 years. At baseline, 15% of exposed workers and 5% of unexposed workers already had mild COPD. After 5 years, the incidence of new COPD cases is 12% in the exposed group and 8% in the unexposed group. The crude relative risk is 1.5 (95% CI 1.1–2.0). However, when the researchers exclude workers with baseline COPD and reanalyze, the relative risk becomes 1.8 (95% CI 1.3–2.5). What does this change in relative risk indicate?