How can we switch our cats cat food?

Our cat loves the cheap cat junk food. Actually we give her Iams, which is a step above purina, et al. However, she only eats the crunchy stuff. Shes interested in the wet stuff, but she just licks it for 10 minutes then walks away. Shes also 7 years old and the vet says that dehydration is causing her bladder and urinary tract problems. He told us that cats should be able to get more of their moisture from food, and told us to get her on a wet food diet. The only problem is, even if we dont give her the dry stuff, she wont eat more than a bite of the wet. Well, shell lick it, but then shell go looking for the dry stuff when she gets hungry. Do we just keep leaving out wet stuff and stop the dry stuff cold turkey? She doesnt like us to mix wet and dry. She sniffs at it and goes on a hunger strike. Any tips?