Why does an enzyme not work as well if its active site is changed?
Enzymes are generally very carefully formed in order to catalyze a specific reaction for a specific molecule. The active site is folded in such as way to just allow the desired substrate to fit into it (like a puzzle piece), so changing the site will make it more difficult for this binding to occur. If the binding is more difficult the reaction will be inhibited, hence the enzyme won't work as well(if at all).
As the enzyme works on the lock and key hypothesis, meaning that it's active site is complementary only to its substrate, when the active sites shape is changed then the substrate will no longer be complementary, this will lead to the enzyme not being able to catalyse the reaction and the rate of the reaction to decrease.