There is no way you can turn off an uniform electric field at an instance. Just like you can not change velocity from some finite value to zero at the same time. There is a transit effect due to the change in field. So you need to specify dE/dt . Otherwise, there is no answer for your question. May be you did not care about the transit effect (may be it is too small), then just assume the net force is zero. You should know what is the answer if net force is zero.