This is what I’ve done a few times, because I’ll move them and need the windows lower or higher.
Figure what height you want the bottom of your window to be, cut that many inches out from the top of your window and move it to the bottom of the window and patch it in place. Then put a 2x4 across the top of the window to brace the top. Then if you want to return the window to the original design just reverse the process.
Works for me.