Change Eligibility Requirements Temporarily?

Plan has typical 1 YOS and age 21 requirement for eligibility, with dual entry dates following.  There is a small match, and an ADP & ACP test must be done every year.  The owner wants to know if it would be OK to amend the eligibility requirements to allow for a new employee's immediate participation and then amend the eligibility back to what it was.  I imagine it would be a much bigger issue if it was an HCE, but let's assume the new hire is an NHCE - can you amend a plan by adding a special entry date to the existing 21 & 1 provision such as 'in addition, all employed on 1/19/23 (the employee's DOH) will become participants on that date'?  It would also allow a couple dozen other employees to enter the plan doing it this way, but they're OK with that.  Thanks in advance for all help.    

Yes, you can do this, and there shouldn't be any need to do two amendments. Most preapproved plan documents have a checkbox in the adoption agreement for special entry conditions, something like "Any eligible employee employed on ____ will become a participant on ____."

Agreed, and someone coming in early is almost never is an HCE these days because of the lookback rule, and not too many come into an employer as an owner unless it's a spouse or other family member.