WPA2 Group Key Renewal -- Will connection be dropped?

I'm not too sure of the technicalities of the key-renewal process, but what I do know is that it should not disconnect or drop any wireless connections made to the access point (unless the clients reject the new key or if the signal strength is too weak to negotiate the new key).

Unless there is a specific need to lower the key renewal timer, 3600 should be fine (it is default for most routers) - as long as you have a strong WPA key.