Updates to Network do not affect sites already created

Sorry I have searched but I can’t seem to find the exact answer that I am looking for. I am just setting up my network so I am obviously making constant changes. One thing I have noticed is that none of these changes ever seem to affect sites that are already on the network.

Example: I decided to make child themes and disable the master theme. This only shows on new sites added to the network.

Example: I decided to disable most of the plugins and let the user decide on which ones they want to activate. However the old sites still show the original plugin layout.

I find it hard to believe that they are stuck with the original set up only. All sites and Networks evolve correct? What am I missing here?