Catalyst definetely changes RxN mechanism; it works by providing an alternative reaction mechanism that competes with the uncatalized mechanism. (find a reaction coordinate in your text book and this should become clear to you).
Now remember that catalyst does not prevent the original reaction from hapening, so the total catalized reaction rate should be the sum of the rates for both reactions. So if your original uncatalyzed reaction rate for a 1st order RxN was: k[A]
now your reaction rate would be k[A]+kc[A]
Remember that catalyst is a kinetic feature; it has to change the reaction rate (both forward and reverse); otherwise why use it.
