Basically, you cant dig tunnels that only hit the highest grade ore and then go back later and dig new tunnels that only hit the more marginal ores because engineers have to plan how to build a construct a mine. As an analogy, imagine if you build a sky scraper and then go to your architect after it's all said and done and say "I want to add 5000 additional square feet to the 32nd floor", that would be crazy, if you wanted 5000 additional feet it would need to be incorporated into the design before the building was built. It's not an adhock go where every you want at any given time sort of thing.