Why not add the SH to the PSP on 1/1/17 and distribute the notice immediately following the amendment that added the SH? As pointed out above, 1/1/17 effective date is not an issue as long as you don't have an existing 401(k) plan. Add the 401(k) and SH features now and you can treat the plan as a "new" plan.
Bottom line, you don't need the to wait until sometime in 2017 or 2018 to add the 401(k) feature or the SH feature. You also don't need to have two plans. The simplest solution is to amend now, distribute the SH notice now, and start as a SH plan on 1/1/17.