If the prospect grants you an audience, don't go in with guns a-blazing. Go in with a pen and paper, and an open mind. Remember that this is a fact-finding mission, not a debate.
Once you've recorded all of your customers' gripes and complaints, take a good look at this list. What do they have in common? Chances are, all of them are fixable! In fact, many of the problem areas may have already been improved. Were sluggish turnarounds a major complaint? Bingo! You've rearranged the equipment capabilities in your plant for faster throughput, and overhauled the shipping department for greater efficiency. Was sloppy customer service enough to make the former client's blood boil? Let them know that you've implemented regular training and review for all personnel in the last few months.