After looking into this a bit more, I will take my guesses. Honestly, the best route might be just use the newer signage regardless. OSHA is trying to push everyone here to use the new updated signage as they are not only up to date, but some are more informative than in the past. If companies are to update, its only a matter of time when companies will be required to do so. You guys can be that one step ahead and not have to go back and update to the newer signs if you do it from the start. Kill two birds with one stone. All the online signage selling sites I am seeing, are all of the newer design. But I bet someday, to comply, the newer signs must be used. For new construction or renovation projects, I know my firm has to use newer codes for buildings when they are adopted at permit. I am sure old signs are grandfathered in perhaps. But once a new building or space is in construction or anything is altered, the newer standards apply which means they loose the grandfathered part because of alteration.
Here is a pdf with some good info.