+1 to what BleeckerSt_Girl said. If the new saddle is not the same height rail-to-surface as your old saddle, or if it is squishier such that you sink down into it more, you'll need to adjust the seatpost height to compensate. Though, I assume you'd notice the height problems more with a road bike; on mtn bikes people (or at least me) tend to adjust the height frequently based on the terrain.