Everything depends on what the parties have agreed in their employment contract.
If the employment contract provides for a flexibility clause
If the parties included in the employment contract a flexibility clause in respect of the work schedule, this means that the employee considers that the work schedule is not an essential element of the employment relationship between him and his employer, so that a change to that schedule cannot be classified as a substantial change detrimental to the employee. The work schedule will then be able to be changed by an addendum, without the employer having to comply with the legal procedure laid down in Article L. 121-7 of the Labour Code.
By making a change to the work schedule, the employer is merely exercising the powers authorising him to ensure application of measures of an internal nature which seem to him to be demanded by the interests of the undertaking.
The decision-making power regarding organisation or reorganisation of the undertaking is not subject to any exception, save where the measure taken has no connection with the reorganisation of the operation and merely constitutes a pretext or is carried out with a culpably frivolous attitude.
The flexibility clause may or may not be considered substantial. It is then necessary to assess the scope of the change.
If the employment contract doesn’t provide for a flexibility clause
- Substantial clause
In the absence of a flexibility clause in the employment contract, the work schedule constitutes an essential element of the employment contract, and any change to that schedule will be regarded as substantial if it is detrimental to the employee.
For example, reducing working hours from full-time to part-time, coupled with a reduction of pay, without the employee’s consent would be considered detrimental.
- Non-substantial clause
By way of exception, if there is a minor amendment, particularly when the employment contract provides for a flexible schedule or when the hours may vary according to the needs of the service, it cannot be considered a substantial change to the detriment of the employee.
In the event of litigation between the parties, it will be incumbent on the employment courts to determine whether the change of the work schedule is to be regarded as a substantial or non-substantial change to the employment contract.