I apologize for the shouting, it's been a hard couple of weeks. And thanks for the support.
Very quickly, the problem with any chain split is double spends. An attacker can spend his bitcoins twice, once using CHECKSIGEX and some script instead of a public key. They can wait for the coins to confirm on the "new" chain, and then they can spend the coins again, using CHECKSIG, on the old chain.
The result would be massive confusion and chaos as those "old" users slowly upgraded and then found their wallets had NEGATIVE balances after the upgrade.