At the instant that the open pipe end changes altitude the water will continue to flow, but that is only due to the tremendous momentum of the water already in motion. The water will now be flowing uphill and will decelerate as expected.

The extream length of the pipe will cause a delay in the decelerating effect due to the length of time it takes for information to travel back to the dam. The speed of sound in water would probebly be the significant gauge here.

Are we getting something for nothing as far as energy? No. It took an imense amount of energy to lift that huge pipe. Ok, lets say it's a massless pipe. It took an imense amount of energy to lift all that water in that massless pipe. You lifted it from somewhere below the waterline in the pool to somehwere above and it is darn heavy.

I would bet that if you plug in numbers, then the energy required to lift that water-filled pipe will excede and energy that "mysteriously" appeared as water coming out the end at that higher altitude. We also might have to take into account the bending of the pipe as you push up on one end and the effect of your pushing travels throught the pipe at the speed of sound through whatever it is made of, which will be less that or equal to the speed of light through vacuum, no matter what the material.

If you run the numbers and I am wrong, then repost and let me know. Very nice question, by the way.
Timothy48342@yahoo.com