Is possible to change the server flying flag without restarting it?

On the server.properties file, there is this configuration value called allow-flight with a default value of false. According to the Minecraft Wiki:

Allows users to use flight on your server while in Survival mode, if they have a mod that provides flight installed. With allow-flight enabled, griefers will possibly be more common, because it will make their work easier. In Creative mode this has no effect.

• false - Flight is not allowed (players in air for at least 5 seconds will be kicked).
• true - Flight is allowed, and used if the player has a fly mod installed.

Can this value be edited without restarting the server? If so, how?

Solution 1:

In vanilla, no.

Changes to the server.properties files always require a restart to take effect, because the server loads its content only at the start. There is no other way around it, unless there is a mod or Bukkit/Spigot plugin to temporarily store the new value until the server is restarted.