Interpretative Guidelines on Regulation EC 261:
"The air carrier should simultaneously offer the choice between reimbursement and re-routing. In the case of connecting flights, the air carrier should simultaneously offer the choice between reimbursement and a return flight to the airport of departure and re-routing. The air carrier has to bear the costs for re-routing or a return flight, and must reimburse the costs for the flight borne by the passenger where the air carrier does not comply with its obligation to offer re-routing or return under comparable transport conditions at the earliest opportunity. Where the air carrier does not offer the choice between reimbursement and re-routing and, in the case of connecting flights, reimbursement and a return flight to the airport of departure and re-routing, but decides unilaterally to reimburse the passenger, he or she is entitled to a further reimbursement of the price difference with the new ticket under comparable transport conditions.
However, where an air carrier can demonstrate that when the passenger has accepted to give his or her personal contact details, it has contacted a passenger and sought to provide the assistance required by Article 8, but the passenger has nonetheless made his or her own assistance or re-routing arrangements, then the air carrier may conclude that it is not responsible for any additional costs the passenger has incurred and may decide not to reimburse them.
When passengers are offered the option of continuation or re-routing of a journey, this must be ‘under comparable transport conditions’. Whether transport conditions are comparable can depend on a number of factors and must be decided on a case-by-case basis. Depending on the circumstances, the following good practices are recommended:
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if possible, passengers should not be downgraded to transport facilities of a lower class compared with the one on the reservation (in the event of downgrading, the compensation provided for in Article 10 applies),
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re-routing should be offered at no additional cost to the passenger, even where passengers are re-routed with another air carrier or on a different transport mode or in a higher class or at a higher fare than the one paid for the original service,
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reasonable efforts are to be made to avoid additional connections,
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when using another air carrier or an alternative mode of transport for the part of the journey not completed as planned, the total travel time should, if possible, be as reasonably close as possible to the scheduled travel time of the original journey in the same or higher class of carriage if necessary,
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if several flights are available with comparable timings, passengers having the right to re-routing should accept the offer of re-routing made by the carrier, including on those air carriers cooperating with the operating carrier,
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if assistance for people with disabilities or reduced mobility was booked for the original journey, such assistance should equally be available on the alternative route."