If they put in a new PS plan, they can keep the TH exemption for the existing plan as long as it continues to consist solely of deferrals + SH match. Then then can exclude this particular individual from both the CB and PS plans, and they won't need to give them a TH minimum.
If that's too much trouble, they could exclude the individual from the existing DC plan going forward; they would probably still need to give them a 3% TH minimum for 2021 (assuming they are excluded from the CB plan), but after that, nothing.