Legally you can change any and all parts of a contract (more or less, law superceeds contracts obv.). It's all just a matter of finding a mechanism or method to edit the contract.
You are not suggesting, are you, that they cannot refuse to accept the changes to the contract, which by itself is said to be a condition for the employment?
Sure they can refuse any changes. And you can refuse any changes. But then you don't have a contract, and then you don't have an employement.
So if they say "We'll give you a job, under conditions X, Y, Z" and you say "I don't like Y", they could either say (a) "Well, OK, we'll offer you a job under conditions X and Z" (in which case you have a job), or (b) "Sorry Y is a deal breaker for us, no deal" in which case you don't have a job.
FYI: Many companies have turned to electronic signing to prevent such modifications to the contract.