(108)Contracting authorities may be faced with situations where additional works, supplies or services become necessary; in such cases a modification of the initial contract without a new procurement procedure may be justified, in particular where the additional deliveries are intended either as a partial replacements or as the extension of existing services, supplies or installations where a change of supplier would oblige the contracting authority to acquire material, works or services having different technical characteristics which would result in incompatibility or disproportionate technical difficulties in operation and maintenance.