Retiling a fireplace
You need to be very careful when removing the tile, as you don't want to break the substrate.  Taking a large hammer to the tiles is a great way to break the brick that is behind the existing tile.

My recommendation would be to get a tile chisel, and use that to break/pry up the existing tiles.  You'll likely be left with a very uneven surface.  Using the chisel, take off as many of the high points as possible.  However, even with lots of attention, you'll likely still have a pretty rough/uneven surface.  To take care of this, use some leveling mortar- apply the mortar in a thin layer, making sure to leave as smooth a surface as you can get with the trowel.  The point being to raise the surface to be flush with the highest points left after you chisel out the tiles.  Let it cure.  After it is dry, use some medium grit sandpaper to make the surface smooth.

Now you have a nice, flat, smooth surface to tile over.  Use a good wall tile adhesive, and tile away.