Our day trip to Mostar from Dubrovnik was just cancelled, and this info might be helpful for anyone considering the same trip. I wish I had more details about our original route and this new route, and I'll update if I get it. Verbatim from our tour organizer:
Due to a new border regulation from the City of Mostar, the route the tour takes to/from Dubrovnik has to be changed and this is causing an additional 2 hours to be spent on the bus. With the already long transfer time, and based on feedback from Group 1 guests, we do not believe this will provide you with the best experience for a day in Dubrovnik and so this tour has been cancelled. We apologize for the inconvenience and disappointment this may cause.