Can COPD test results change over time?

Yes . Pulmonary function tests typically "deteriorate" as you get older. If a person has impaired pulmonary function as a result of copd, the results can also deteriorate over time. The rate of this decline is variable and is affected by certain variables including tobacco use and genetics. The test results (if performed in a steady state) typically do not improve over time.
Yes... Pulmonary function testing is typically used to measure the presence and severity of the chronic airway obstruction. The severity of obstruction can change over time. The obstruction will worsen with disease progression over time, but can also sometimes improve. For example, if the patient stops smoking and the airway inflammation is lessened, the obstruction measured may decrease as well.