Safe Harbor Plan - Exclude HCEs beginning of the year

Can you make an amendment to exclude HCEs from being eligible for a Safe Harbor plan (effective 1/1 and amended at least 30 days before beginning of the year)? These HCEs already met the plan's eligibility requirements.

Thanks!

My understanding is that you can amend to exclude them prospectively and that the document probably requires a 30-day advance notice to do so, even though they are HCEs. But check IRS Notice 2016-16 to see if you agree.

Are you actually trying to exclude existing participants from participating in the plan, or just excluding them from being eligible for a Safe Harbor Contribution?