Prevent existing Post text from being modified during Post Edit

Hi All,

I need your help in solving a problem I have.

I would like to let my blog authors to add some content to their and others Posts BUT don’t want them to modify content that already exists in this Post.

Does anyone know any plugins for that or can give me a clue? Or just tell me “Forget It” if it’s not possible. I don’t need any complex restrictions on specific user level.

Thanks for your help!

Cheers,
Sergey.