Put the new filter in with the old filter so it starts to build up the helpful bacteria. Or squeeze bacteria+ from old media (floss etc) into a container with tank water and the new filter to allow the new filter to start growing. If you just take out the old filter and replace with brand new, there isn't enough good bacteria just in the substrate to keep up the cycle, therefore you start a brand new cycle. It's a good habit to get into, when you plan on changing the old filter, just pop a new one (rinsed) in with the old for a few days.