Image generator from existing Media instance (of type video)

[jozeflambrecht]

Hello,
I have my existing $media instance and I would like to generate a thumbnail at $seconds.
I tried:

$media->manipulations = [
  'thumb' => [
    'height' => '250',
    'extractVideoFrameAtSecond' => $seconds,
  ],
];

$media->save();

But get this error:
Can not convert image. Unknown operation extractVideoFrameAtSecond used

[jozeflambrecht]

Not possible with existing? Conversions only possible when adding?