Some may say that you cannot merge a DC plan into a DB plan (or vice versa). I believe that is a misreading of IRC 414. However, it does appear that to do so, you must first treat the disappearing plan as if it were being terminated (and my statement may be an oversimplification). Thus, you have to vest the participant.
If there is also a forfeiture account waiting for allocation, then allocate it .
On a practical level, with only one affected participant, it seems the sponsor would gladly opt for the simplest approach.
Other comments?