Why do you need to recompute the entire block chain. At the moment, the block chain is in some state. If you control the majority of the processing power, you c

Why do you need to recompute the entire block chain. At the moment, the block chain is in some state. If you control the majority of the processing power, you can devote it all to expanding the chain from that state. Because you have more CPU, your version of the block chain would replace the old version when you choice to release it. This would not let you re-write history from before you started mining, but gives you complete control of events after you started.