Our certificates don't lapse in NJ, but the PD requirement stays in effect. Nice because we don't have to pay to have our certs every two to three years, but the trade off is that NJDOE expects you to be ready to use that cert and be up to date at the same time. NJ also doesn't usually pay subs any differently, but some districts might pay a little more for a certificated teacher, especially if the length of assignment stretches past the 20 day limit. I don't know if being a certified teacher would remove that limit, but it would at least make it possible for the teacher to stay on as a long term sub/replacement as long as the certs matched up. OR, some retired teachers simply can't stand to let their brains atrophy. Once a lifelong learner, always a lifelong learner.