The easiest solution is to complete the raising of platforms half-by-half. As the improved half-platforms come online, start equipping the trainsets half-by-half as well. GO already does this - opening half of a train at a time - whenever any major platform construction is needed, and so there would really not be any new processes required to be introduced or taught.
It really doesn't need to be any more difficult than that. No need for movable, foldable steps (that need to first be engineered, then built and tested, and then installed).
Dan