Block 200001 contains a hash of block 200000. This verifies that the block 200000 has not been changed. A replacement for block 200000 would stop at that block, and not be a part of block 200001's chain.

The Bitcoin client will accept the "longest chain", which is the chain with the most work behind it. Because the replacement block's chain ends at 200000, it is shorter than the original block 200000's chain and it will be rejected.