This is pretty interesting. Ideally I was thinking of being able to do a head to head test.
IE. You could convert a single node to a compressed LCS node. It would take reads and writes, but you would not do any repairs, or joins, or moves or possibly any other schema changes. At some point you would evaluate the performance vs another node in your cluster, look at your performance and make the decision. If you want the change to happen across the cluster or be undone.
I know my request is asking something to the effect of 'Help me test in production', But coming up with hardware and a system to do an actual test of production load is difficult. If we were able to restrict a schema update to one node we have a way to mitigate risk. For a major change like compression or LCS that will drastically reshape a lot of physical data on disk and could drastically change performance hopefully in a good way, but if it did not work out well, performance could suffer for a long time while the underlying data tables rewrite themselves and you get back to where you started.