We've had several moments where we've had to stand our ground on our contract. You can do it politely, but, in my experience, you have to stand by your contract. There are several people who are of the opinion that we should bend every time they want to change our contract, but I very much disagree. Our contract is the only thing we hve to go by. If they're able to change it at will, then what do we have?
On several occasions, I've very politely, but very firmly, told them my contract is not open to change and that they must wait for the next contract before they can make any changes.
I also think it's important to not allow the contract to be changed because, if you let the changes happen, then it becomes a precedent and they think they can change the next teacher's contract.
The idea of contracts is a new idea to many Koreans because it's not how they usually operate. But since we're working in a cross-cultural situation, they have to realize that a contract is binding. We work hard to keep our end of hte deal and they must also work hard to keep their side.
That's just my two cents!