If travel trade customers arrive with a coach that is larger than the details on their reservation, will they still be able to board? 

Providing space is available you will be able to board and an excess charge for the larger vehicle will be taken.   

If space is not available travel won’t be possible. You will be offered an alternative sailing time or date. You can arrange with port staff to change to an available sailing time, or have your tickets refunded less the administration charge if alternatives do not suit.