Hi Joris,
One more question regarding this. I know is an old topic. If there's an over-layering modification, that adds a new field in a standard table. (This is in Production environment). Now we are working in a extensible solution in Development environment. Let's say we deleted the field from the overlayering model, and created a new one in a new model (because as you say we can't move the customization in that scenario). What will happen when we deploy the solution package into Sandbox? will the data be deleted from the field / even though the field still exists in the table but now in a different model?