Everything depends on what the parties have agreed in their employment contract.
If the employment contract provides for a mobility clause
If the parties included in the employment contract a geographic-mobility clause, this means that the employee considers that the place of work is not an essential element in the employment relationship between him and his employer, so that a change to that place cannot be classified as a substantial change detrimental to the employee.
The place of work 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.
For example, it was ruled that an employee whose place of work was transferred to another Member State and who had a mobility clause in her contract and for which there were no details as to the disadvantages of such a transfer, that this change was not substantial and that there was no need to apply the procedure provided for by Article L. 121-7 of the Labour Code.
Furthermore, the fact that, after the transfer of a company, an employee is called upon to work in a border area outside the Grand Duchy does not necessarily constitute a substantial change to the contract where a flexibility clause exists.
If the employment contract doesn’t provide for a geographic-mobility clause
In the absence of a geographic-mobility clause in the employment contract, the place of work constitutes an essential element of the employment contract and a change to that place will be regarded as substantial if it is detrimental to the employee.
The view has in particular been taken in the case law that a change of a Luxembourg employee’s place of work to Paris is to be regarded as a substantial change by reason of the distance which separates the two cities and the disadvantage which the change represents for the employee.
In the event of litigation between the parties, it will be incumbent on the employment courts to determine whether the change of place of work is to be regarded as a substantial or non-substantial change to the employment contract.