Please explain to me how this shows it "changing course?"
[link to h.dropcanvas.com]
For reference, those are the raw frames that make up your movie.

Oh, you mean because you see the stars moving in a sine wave motion relative to the comet? Well there's this cool thing called "parallax" which is especially evident here because the Hubble Space Telescope which shot these images is orbiting earth. It's constantly moving around earth in low earth orbit, and it does so fairly quickly. On May 8th, when the images in your video were shot, ISON was about 4.34 AUs from earth. That's about 649,254,759 km. Given that Hubble's orbit is about 550 km high, and given the diameter of the earth, the maximum baseline Hubble's orbit can form for parallax is about 13,842 km (earth's diameter + Hubble orbital altitude x 2). That works out to a maximum parallax angle of about 4.4 arcseconds. By comparison, Hubble's WFC3 resolution is .04 arcseconds per pixel:
[link to www.spacetelescope.org (secure)]
4.4 arcseconds is the maximum amplitude of the sine wave we would therefore expect the stars to form. Naturally the observed parallax can be less than that depending on the inclination of Hubble's orbit relative to the path of ISON's motion in the sky at that time, but the maximum is 4.4. That works out to about 110 pixels at the native resolution of the CCD. The raw images appear to have been 2x2 binned for this acquisition, the measured amplitude at the half resolution size is 14 pixels, so the amplitude of the sine wave at the native resolution would have been about 28 pixels in this case by my measurement, well within the expected amount of parallax.

So no, the video does NOT show ISON "changing course," but you did give me some brilliant ideas for my own response video.