In part, this unchangingness follows from the claim that the Gods strictu sensu are perfect: literally, they are complete, lacking nothing that they could have or be. If they were to change, that would either be a change that made them better (in which case they were less than perfect to begin with) or a change which made them worse (and thereby compromised their perfection).

Our conversing with the Gods (or not) does imply a change in state, but it's a change in *our* state, not in theirs. Their providence is always present, but we can change to become more or less receptive.