The "plan" was to first just kill Dent--or at least target him--in order to lure Batman to him, so that the Joker could kill him and get his money. The contingency would be that, if the plan fails (i.e. Dent lives and Joker ends up getting caught), his men would kidnap Dent and Rachel as a way of forcing Batman and most of the cops to leave the MCU, so Joker could escape.
Joker doesn't really care about Dent, he's just a means to an end in getting Batman. And like someone else said earlier, the Joker's motivation ends up changing right around that point. In the beginning, he just wants to kill Batman for money; when he realizes how much fun he's having, he decides that instead of killing Batman, he's just going to keep screwing with him over and over (which is why he switches the addresses).