This is the text from the link justanotheradmin posted. Notice 2016-16 doesn't say "or vice versa" in the final bullet point below, but the commentary on the website does.
Prohibited mid-year changes
The Notice provides the following list of “prohibited mid-year changes” that may not be made to a safe harbor plan, unless the change is required by applicable law or court decision.
A mid-year change increasing the years of service for the vesting schedule for a safe harbor plan consisting of a Qualified Automatic Contribution Arrangement (QACA);
A mid-year change to reduce or narrow the group of employees eligible to receive safe harbor contributions; however, this does not limit the ability of the employer to amend a plan mid-year to change eligibility service crediting rules or entry date rules for employees who have not yet become eligible to receive safe harbor contributions;
A mid-year change to the type of safe harbor, for example, a change from a traditional safe harbor to a QACA, or vice versa;