Hi Jmcculley! Changing the position of the violin thumb will definitely effect muscle memory and accuracy of pitch. You will have a period of time that requires adjustment and relearning but it is necessary and worth it. I haven’t seen Michael’s video, but I do advocate thumb opposite the first finger. More specifically, with the left hand turned with the palm looking at you, the thumb closes up next to the base of the first finger, but remains straight. For most people, this results in a ‘little V’ where the violin neck touches the thumb and base of first finger. There are acceptable variations of this position of the thumb, and if we look at the greatest players of our day, we’ll see as many unique approaches to thumb positions as we find shapes and sizes of thumbs. Even so, there are ‘rules of thumb’ (pun intended) that are understood and then professionals break them for various reasons, most often temporarily.
Here’s a great left hand thumb position. Notice the thumb is straight, but relaxed. It sits opposite the base of the first finger and just behind the first finger tape. Almost none of the thumb sits above the fingerboard.
I know it’s frustrating to change something in your playing after you’ve put a lot of time into it a different way but let me encourage you to stick with it! And take heart! At least no one asked you to do this!