1. Contracts and framework agreements may be modified without a new procurement procedure in accordance with this Directive in any of the following cases:

(a) where the modifications, irrespective of their monetary value, have been provided for in the initial procurement documents in clear, precise and unequivocal review clauses, which may include price revision clauses, or options. Such clauses shall state the scope and nature of possible modifications or options as well as the conditions under which they may be used. They shall not provide for modifications or options that would alter the overall nature of the contract or framework agreement;

(b) for additional works, services or supplies by the original contractor, irrespective of their value, that have become necessary and were not included in the initial procurement where a change of contractor:

(i) cannot be made for economic or technical reasons such as requirements of interchangeability or interoperability with existing equipment, software, services or installations procured under the initial procurement; and

(ii) would cause significant inconvenience or substantial duplication of costs for the contracting entity;

(c) where all of the following conditions are fulfilled:

(i) the need for modification has been brought about by circumstances which a diligent contracting entity could not foresee;

(ii) the modification does not alter the overall nature of the contract;