well, maybe yes maybe no
you can certainly change eligibility and even someone who was previously eligible could become ineligible.
I suppose in the adoption agreement
Special Participation Date
a. [ ] Allow immediate participation for all Eligible Employees employed on a specific date. All Eligible Employees employed
on shall become eligible to participate in the Plan as of _________
.............
so lets say you only wanted this prospectively, a date of 3/1/2016 could have been filled in, and thus have the effect of a 1 month wait up until 4/1/2016.
on the other hand, some document language simply says
something like "Anyone who was a participant prior to the effective continues to be a participant, bleh, bleh, bleh"