My guess is:

 - You can't change the genesis block, as it's hardcoded into the general release and future blocks wouldn't be accepted at all
 - You would need to know the pass phrases of a sufficient number of the 72 original accounts to forge the remaining blocks with at least the same total difficulty as the current chain
 - Anyway, the release client has a restriction that it will never switch onto a fork >720 blocks old (not sure about this one?)