Threshold User Authentication is widely used to provide distributed authentication service in Wireless Mesh Networks(WMNs). In a (t, n) threshold authentication scheme, secret authentication keys are shared among n mesh authentication servers (MAS) and any t out of n MASs can collaboratively provide the authentication service. The existing threshold authentication methods usually assume that MASs are static in the system. That is, t and n do not change after system initialization. However, in WMNs, MASs often join or leave the network freely. In this paper, we propose a novel threshold key redistribution protocol, aiming to cope with dynamic change of the authentication group. We also discuss the key redistribution protocol that can tolerate compromised MASs in the network.