Is it obligatory for the congregant to wait for the Imam to turn and face the congregants before moving or changing his sitting posture from what it was before the Tasleem, or is there no harm in moving before the Imam turns?

There is no harm for the congregant (ma'mum) to change their sitting posture before the Imam turns his face towards the worshippers. This is because what is disliked is to turn away by standing up from one's place, for fear that the Imam might remember an oversight. This waiting (until the Imam turns) is recommended, not obligatory.