Let’s say you do yoga at company X. You have registered for 1 year starting in January and you have paid in advance. In April, you are told that the schedule will change and that your course will now be at 6 pm instead of 7 pm. This is a problem because with your work schedule and family obligations, you cannot be available that early. Can the owner of company X impose a new schedule?
Unless it was provided for in the terms of registration, the answer is no. No one can change the terms agreed upon once the contract is in progress. The company is committed to giving you yoga lessons that day of the week at this specific time and you are committed to paying the agreed price. Any changes must be made by mutual agreement. What to do then to sanction this defect? You could theoretically ask for specific performance and require that you be given the agreed yoga lessons, but it might not be the best solution in this case, for obvious practical reasons. It would be more appropriate to ask for the cancellation of the contract and a refund, in proportion to the services that you could not receive (in this case, 75%).
The example above is simple and can be transposed to any type of contract. When a contract is underway, the contracting parties must be able to expect a certain stability, or face a penalty.