That is the key. Typically, unvested balances forfeit at the earlier of distribution of vested balance or 5 consecutive one-year breaks. If you have unvested balances remaining for participants who have been gone longer than five years then you could have a compliance issue - operational defect for not following plan document that may have required forfeiting and re-allocating (or reducing ER contributions) in a specific time frame. You should make sure that is clean before terminating the plan. Then, anyone who has not forfeited unvested balances under the terms of the plan must be fully vested upon plan termination.