Can I move heating vents below a window from the wall to the sill?
You only use heating when it is cold. When it is cold, the windows will be cold. Running the heat up along the cold windows will allow the windows to suck the heat out of the heated air and to the outside. As you have it now, the air is being pushed out into the room to circulate and mix with the other room air, warming the space, not the windows. Eventually the warm air gets back to the windows and gets chilled against them, but it is not circulating as fast. Doing what you propose will likely accelerate that process. I would not do it, your heating bills will likely go up.
What is your reasoning behind this?