How much electricity can plausibly be saved this way? And how does that compare to the energy required to reprogram and possibly renew all these signs?
According to the source linked in the top comment, we are looking at a total consumption of 113.000 MWh per year for those displays. One third of that is at least around 30.000 MWh of saved energy.
Well, in that case we shouldn't improve anything as long as children are starving to death in this world. That should be our only and biggest "bug" to fix. Feel free to recommend another one, but we'll pause all other improvements meanwhile. ¯\_(ツ)_/¯
Digital billboards use thousands of watts, overall, that's several times the energy consumption of a typical household, the power bill is thousands of euros a year.
So I guess that's worth it, even if some additional hardware is required.
About third of what was used before.
> And how does that compare to the energy required to reprogram and possibly renew all these signs?
That's cost of doing business and a one-time thing.