The short answer here is that it depends on how significant the change is going to be. Employers obviously need some degree of flexibility from time to time, and a well-written contract will build that into the agreement. If you have that flexibility built in, and want to make slight changes that do not significantly impact an employee’s wages, benefits, or duties, then you can do that without any re-writing.
If you are looking to make a significant change though then a new contract is probably in order. This is where new contracts can present a great opportunity to adjust the terms of work for existing employees. A slight salary increase or the introduction of a new benefits plan can serve as fresh consideration in exchange for changing the terms of the contract.
If the employee is going to view those changes as unfavourable, then they may not agree to the new contract. That then presents an opportunity to offer that employee working notice of dismissal- the new agreement will be in place as of X date (a date discussed with your lawyer), and if that employee does not agree to the new terms then they are no longer employed.
