You will have to attach them only once to base image. They will be updated automagically with new images 
Hi @pasqdnik. Yes I assumed that was how it would work but when I re-based from Silverblue 31 to the 32 Beta I and others hit an issue (Fedora 32 pre-beta (including issues & workarounds) - #16 by joefidler). Probably (hope) it will go bit smoother with released versions of Silverblue and the RPMFusion repos.
Is there a post, or do you have some notes? on how you got codecs working on Silverblue? I am thinking of migrating from Workstation 32 to Silverblue 33 after release and codecs one of the things I haven’t sorted out yet.
There are flatpaks of some common codecs (see Codecs and other functional overlays - #2 by mpphill2) along with the Firefox flatpak from Flathub, which also has some built-in codecs. Up side of this approach is that you may not need to use rpm-ostree to layer the rpm-fusion repos, which will make re-basing Silverblue versions even more seamless.