I don't think that changing priority is actually feasible in Factorio due to the way things are programmed. I instead finally found a workaround by making a day/night sensor: you place a solar panel, an accumulator, and an electric pole next to each other on their own electric network, then add something which will consume an ideal amount of power such that it will allow the accumulator to fill in the day but drain at night. (Depending on your mods, solar panels, accumulators, you may need more of one or the other.) With vanilla settings, you can use several lights as they even conveniently shut off during the day. Then hook up the accumulator to a circuit network (attach a red or green wire from accumulator to electric pole) and then anything else on the same electric network can now use the accumulator charge level to determine whether it is day or not.
So then to shut off steam turbines during the day, you just hook the steam turbine power grid to the main power grid through a power switch which is on at night (example condition: [A] < 25). Pro tip: read the charge level from your primary accumulator banks instead, and don't let the turbines run if the accumulator bank isn't out of power yet. To prevent the rapid stop-start-stop-start fluctuation, don't use a power switch and instead connect the inserters which insert fuel to the turbine system, there will be a delay between when they start inserting fuel and when you get power from it.
