The constellations upon which the western zodiac is based have not changed their positions. They appear to have done so because Earth wobbles around its axis once every 22,000 years, thus seeming to throw off the established dates by about a month. The dates have changed for the sidereal zodiac but not the tropical zodiac, which is the one most commonly used.
The sidereal zodiac is based on the sun's position relative to the 12 commonly accepted constellations. These include Aries, Taurus, Gemini, Cancer, Leo and Virgo. They also include Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces. Although Ophiuchus falls between Scorpio and Sagittarius, it is not used. The Earth undergoes precession around its axis due to the combined gravitational pull of the sun and the moon. This causes the constellations to appear to shift. The sidereal zodiac is used by astronomers.
However, the tropical zodiac system, used by astrologers, assigns zodiac signs based on the seasons and equinoxes. From the perspective of the Northern Hemisphere, the summer solstice occurs between Gemini and Cancer; the winter solstice between Sagittarius and Capricorn; the vernal equinox between Pisces and Aries; and the autumnal equinox between Virgo and Libra. Each sign has a month of prominence, and the seasons do not change.