(109)Contracting authorities can be faced with external circumstances that they could not foresee when they awarded the contract, in particular when the performance of the contract covers a long period. In this case, a certain degree of flexibility is needed to adapt the contract to those circumstances without a new procurement procedure. The notion of unforeseeable circumstances refers to circumstances that could not have been predicted despite reasonably diligent preparation of the initial award by the contracting authority, taking into account its available means, the nature and characteristics of the specific project, good practice in the field in question and the need to ensure an appropriate relationship between the resources spent in preparing the award and its foreseeable value. However, this cannot apply in cases where a modification results in an alteration of the nature of the overall procurement, for instance by replacing the works, supplies or services to be procured by something different or by fundamentally changing the type of procurement since, in such a situation, a hypothetical influence on the outcome may be assumed.