I am guesssing the tile is their to help keep your pond full of water during normal and dry years. Flushing water through your pond does help a little in reducing the nutrient load in the pond but it can also aid in silting it in. If you are intersted in re-routing the drainage you should check with your neighbor to see if they have any objections, otherwise there could be might be liability isues if you end up flooding and damaging your nieghbors property. I would first check with your nieghbor and then check to see what state and local laws might apply regarding changing the drainage route.