A couple of Q&A items from the 2012 ASPPA Annual Conference DC Q&A session are relevant to this discussion if it involves a SH plan.
#39 - SH 401(k) fails 410(b). Question is does the -11(g) amendment to correct the 410(b) failure violate the mid-year amendment prohibition for the year the amendment is adopted? Answer is no and the IRS agreed.
#37 - SH 401(k) that only covers salaried employees. Question is can they amend mid-year to make hourly employees eligible? IRS answer is yes, provided the amendment doesn't adversely affect those already eligible.
I'll also point out that there are no rules in 1.401(k)-3 or 1.401(m)-3 that apply to plan eligibility requirements. The SH mid-year amendment prohibition applies to provisions that satisfy rules in those sections.
So, even if it is a SH plan, you should be safe in making the amendment be effective for both 2014 and 2015. However, you may not want to have it effective for 2015 if the plan will pass 410(b) for 2015 without the amendment. Amending now to retroactively bring in previously excluded employees means that they become improperly excluded for that retroactive period. The correction for that is for the employer to make a corrective deposit. Why do a correction for 2015 unless you have to?