Having >50% hashing power doesn't let someone change the rules; that requires >50% of all nodes to agree.
There are some truth to this, there was an incident early in development it was discovered that some hackers had 20 something billion Bitcoins in their wallet, the system back then didn't account for more than 21 Million coins in existence, so couldn't do anything.
The developers re-wrote the client and asked all miners to upgrade, the 20 billions coins the hackers had became obsolete.