There’s no grandfather exception to the ADA; the facility is expected to “remove barriers to access” whenever “readily achievable.” This language does cause a lot of uncertainty, since readily achievable is usually dependent on resources and cost, or on structural barriers. But in the case of parking, which unlike prohibitive staircases or narrow doorways can be comparatively easy to change with re-striping and signage, there should never be any reason not to comply with the law.