Yes, it is true. Ortho-l lenses are similar to RGP lenses and they are most effective at the first year. If you wear them overtime, the lenses will be out of shape and impair the effect of vision correction.
In fact, ortho-k lenses can be used for one year. If they are out of date, the shape will change. Then they can't correct your vision effectively. So you need to replace your ortho-k lenses every year.