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 there is no space available, then travel won’t be possible. You will then be offered an alternative sailing time or date. You can arrange with port staff to change your sailing time if space is available or to have your tickets refunded less the administration charge, if no alternative is suitable.