That actually changes it a lot less than raising and lowering
significantly. HF yagis a wavelength or two above ground don't have
laser-like beams. The elevation pattern is set up substantially by
ground reflection, not only the antenna's free-space elevation pattern.

Check out

http://n3ox.net/pictures/20m_yagi_el.jpg

for an illustration of what happens when you tilt a 5 element 20m yagi
up toward the sky. I don't remember how high it is; I think it's about
1 wavelength high. The traces are for 0, 20, 40 and 60 degrees
inclination.

73,
Dan