Glad they are your client and not mine!
@Nate Smentions amending the plan to be a 5% safe harbor match, but I believe you had to make that amendment by the last day of the following play year, which would have been December 31, 2021.
I think you will run into issues even if you say the extra 1% is a discretionary match. Basically any participant who deferred less than 5% likely did not receive the 1%. Or at least not the full 1% if they deferred between 3% and 5%. Have to also consider if the discretionary match is based on a pay period or plan year. If plan year, you may have catchup contributions to consider as well. Not to mention the headache if a participant has left in the meantime and already rolled their funds out of the plan. May be worth talking to ERISA counsel on this one.