What is the role of this Zener diode in a LED string? (Old traffic light restoration)

[Paul_B]

@TimewaveZero

To achieve a 24-30V running of this LED PCB I'll need to de-solder all resistors from the board and replace them with a lower value corresponding to a determined lower input voltage. Luckily everything is well accessible. Are there any other things I have to watch out for (or take into account) when attempting this conversion?

Yes, a chain of eight green LEDs in series may be difficult to power at 24V.

Better to provide the proper (original) design voltage unless you want to do a lot of re-wiring.