Safe Harbor Mid Year Retroactive Amendment

A safe harbor match 401(k) plan currently has a six month eligibility (no hours per month requirement).  

Due to some part time students who work on an as needed basis, the Employer would like to amend their plan to require 200 hours per month as part of their 6 month eligibility.  They would like this amendment effective date 1/1/2019.

I believe the amendment is permissible, but only PROSPECTIVELY with a 30 day notice.  

A coworker believes the amendment can be effective retroactively to 01/01/2019 because it will only apply to those hired on/after 01/01/2019.  

I'd appreciate some input.  

Your details are not clear, but consider the following:

1.  You can't amend mid-year to make someone who is eligible to receive the safe harbor contribution become ineligible for it.  You can amend the eligibility requirements going forward provided they only apply to people who are not yet eligible.  Notice 2016-16 III D 2

2. A retroactive amendment that makes someone ineligible for a contribution they have earned the right to for that plan year violates 411(d)(6).  1.411(d)-4 Q&A 1(d)(8)

3. 200 hours per month for 6 months would require at least 1,200 hours for entry.  That averages a little over 46 hours per week.  It can't be the plan's only eligibility requirement because you can't require more than a year of service for the 401(k) portion of the plan.