An unwilling transformation is the only way I can see it being done. High elves chose integrity over their well-being, they wouldn't suddenly choose to turn into void elves in mass without betraying the very reason they split from blood elves in the first place.
But such mass transformation hasn't happened yet, so it's still speculation.
And people need to understand that if high elves are turned into void elves, the void elves themselves will lose their identity. You will suddenly have tragic void elves seeking a cure or unwilling to use the void as the majority of the race.