I believe the BB live on your biowheel. What filter cartridges are you thinking to replace? You don't want to replace anything that harbors your BB.
I would just replace the mechanical filter cartridges, in the back of the filter. What makes me WANT to replace them would be because of all the possible gross dead cyanobacteria that built up in the cartridges (and thus I feel like they could decay and cause possible problems?). Why I DON'T want to replace the cartridges is because I already replaced the cartridges a day after the big cleaning. So they are quite fresh/new haha.