We are going to need to re-queen one of our hives too.  She is getting a bit slow and the bees do not seem to be caring for her as diligently as they were.  Sometimes the hive will decide to start making Queen cells and the new Queen fights the old Queen, but I think it is better to be pro-active and find a good genetic queen.  When you have the new Queen, you would find the old queen and killer her (I know...this would be hard for me too....I love my bees!!)  Then you would introduce the new Queen into the hive just as you did before. 

The other way would be to kill the Queen that you have (just make sure that you have lots of new eggs in the frames.  Once the old Queen is gone, the hive will make a new queen.