If someone had >50% hashing power, they couldn't just start rewarding themselves more than 50 BTC, as those blocks would be rejected by honest nodes. No?
All you can do is rewrite the last x blocks. ie. edit and reverse transactions. If you reverse your own recent payments you can double spend and if you reverse other peoples it will obviously cause mayhem and totally undermine the whole bitcoin system. You cannot change the block rewards, give yourself free coins, or steal anyone elses money as you will never have their private keys to sign transactions.