A foam filter like the one in the Fluval serves two real purposes. One is to mechanically filter dirt and debris out of the water and the other is to provide a bed for the bacteria to colonize. Because it will clog with the very debris it is suppose to filter out, it does need to be cleaned but as long as you are careful to rinse it out well in old tank water (to avoid exposing the bacteria to chlorine or chloramine in tap water) you will revitalize the filter's mechanical filtration abilities while preserving the bacteria colonies.
If the filter gets gross enough that you feel you must replace it, I would suggest leaving the old filter cartridge in the tank for a few weeks while the bacteria colonize the new filter.