The NEC can change when the reselection process happens, and the terms of the reselection process, if it chooses. I suppose it could choose to do this soon, but it won't.So he can call for reselections. You've just decided it's not the right time.
It isn't a simple matter of Corbyn's allies now controlling the NEC btw, the recent elections have only changed the balance of power by two (or maybe 3?).