When it comes to antonyms for the phrase "changes position," there are a few different options. The first is simply "stays still" - if something is not changing its position, it is staying still or stationary. Another antonym could be "maintains position," which implies that something is intentionally staying in one place. "Remains fixed" is also a possible antonym, although this carries the connotation that the object in question is unable or unwilling to move. Finally, "keeps constant" is another option - if something is not changing its position, it is maintaining the same location over time.