Also there are some easy ways to dismantle some of the unnecessary data, like holes, terrain changes, consequences of mining.
E.g.
- new mining units could do some tile clean up and fill the holes, delete the tunnels
- they can introduce "weather" changes. In RL the hole in the ground will disappear after some time, due to the weather, earthquakes, ... So they can slowly start to simulate the same effect and slowly delete this mess.