Chumchuck – if you downgraded the date from say dinner to just a soda or coffee, her parents surely will ask her where you went, and if they thought it was going to be a very different type of date, she WILL be offended, when she realizes why you did it.
My son went out on blind dates with girls who were not for him l’chatchilah, but who came highly recommended and he often knew that within moments of picking them up. He nonetheless ALWAYS made sure that they enjoyed the date, followed through on whatever had been planned, and only in one case was he ever sorry that he did not simply cancel the date altogether. Even if the particular girl or guy are not for each other, they might have friends or relatives who would fit the bill so why give them a bad impression of each other. In any case, one should always be a mensch, regardless.