Lots of things can happen between the interview and starting a job, the original project could be cancelled, a higher priority project may have someone leave suddenly and need a replacement more urgently, they may still be planning to use the technology you are interested in but the project start got delayed by a couple of months and they put you in the other spot to avoid losing you, etc. They might even have put you on a legacy project to get you some understanding of the current problems in preparation for a future project to replace some of all of the old one.
These things happen after you have worked somewhere a while too, so don't get too attached to what you are going to be doing. We all occasionally get put on projects that we would prefer not to work on. The needs of the organization are paramount in assigning work. You can, of course, politic to get the assignments you want (which you should always do). But even then there are no gurantees. Sometimes things out of your control or even the organization's control happen.
The best tactic is to talk to your boss about where you want to be and how to get there. Ask what happened to the project you thought you were being hired for. Your options are different depending on what caused you to be assigned in a place other than that which was discussed.
And in the meantime do a good job at whatever they have asked you to do. You have more leverage when they want to keep you than when they don't care. No one is going to care about the wishes of a whiny prima donna who thinks he is too good to do the job we gave him to do and who blows off the work. And there is something to be learned from any job and any task. And start learning the business of the organization (you can't have too much domain knowledge and who knows this chance to learn the legacy product might turn into leading the redesign in a year or so) and the political structure. To get the assignments you want now, you will have to play the political game, so use this time to scope out the right people to know and to talk to about future assignments. Make people want to fight to have you on their team.
The alternative is to look for another job. This will work once or twice in your career, but use it sparingly, spending only a short period at every job will get you labeled as unreliable, unrealistic, or difficult to work with.