Not exactly. Your employer does have the power of some flexibility within the employment relationship. Most employment contracts that were drafted by a professional include a line, usually in sections about hours of work and job duties, that says that things may change from time to time. So if your employer makes a small change and asks you to do some additional work one day, they likely have the freedom to do that.
What your employer cannot do, though, is completely ignore the terms of your contract and start making up their own rules. For example if your contract says that you’ll be paid bi-weekly and that you’re expected to work 40 hours per week, your employer cannot decide to suddenly start paying you monthly, or spontaneously make you work 60 hours per week without any overtime pay. That would show that the employer is acting as though they’re no longer bound by the contract (see below).
