Convert a Dusk-to-dawn outlet device to Dawn-To-Dusk?
A photocell transistor (aka optocoupler) or photodiode is not a perfect opposite of a photocell resistor (aka LDR). Going this route would require characterising the LDR and then carefully designing an optocoupler circuit to do the opposite. Sounds tricky.

Instead, I suggest leaving the detection circuit alone. It sounds like it is doing what you want - detecting the two switch points based on ambient light. All you really care is that the switch-off event becomes a switch-on event and vice-versa.

If that really is a SPDT relay, then it already has the function you need. A SPDT has a normally-open (NO) contact and a normally-closed (NC) contact. At switching time the relay will close one contact and open the other. The mains power will be connected to one of the contacts. All you need to do is connect it to the other, so every time the switching event occurs, the opposite switch action takes place.

For example, the current circuit might look like this:







simulate this circuit &ndash; Schematic created using CircuitLab

and all you need to do is change it to this:







simulate this circuit

Note the numbers in the schematics are not important, just the change in wiring on the relay.