Solo 401(k)

Sole Prop established a Solo 401k a few years ago.  All was well as he had no employees

In February of last year (2021) a FT employee was hired  Unfortunately, the Plan had both immediate eligibility & vesting  for PS contributions.

May 2021 - owner amended the Plan to a SH 401k and added a vesting scheduled for PS contributions - an effective date of 1/1/2021

The sole FT employee has since severed employment 

Question?

How is vesting calculated? 

Is the former employee 100% vested (since there was no vesting scheduled when he became eligible) or does he follow the schedule (since the amendment was effective 1/1/21) prior to his DOH?  

All help is appreciated. 

Thank you

EE is 100% vested.

Bill is 100% correct. He became eligible on his date of hire and immediately 100% vested. Any reduction from 100% would be a prohibited cutback under 411.

Very helpful. Thank you