A teacher who has withdrawn from the fund as set forth in this chapter, by returning to teach in a public school or state institution of this state, may regain service credit for prior teaching by making the required payment. The required payment is the purchase cost on an actuarial equivalent basis. If the teacher returns to teach in covered employment after June 30, 2008, the teacher becomes a tier two member regardless of whether the teacher repurchases service credit earned while the teacher was a tier one member.