Question Allow editing but forbid editing on original text

Hello everyone, I would like to allow my users to edit their posts (for unlimited time) but forbid them to edit the original text they wrote in the message.
Basically the editing would only "add" the new text below the old one.
A kind of "update", is often used on the first post of important topics.
Is there a way to do that?

Simply put, the answer to your question is no.

There is no mechanism for storing multiple versions of a single forum post and displaying all of those versions simultaneously so that you saw what was originally posted and the various stages that it may have undergone subsequently. Perhaps this kind of feature is appropriate in a semi-legal or office environment where there is a need to track changes - e.g. software like MS Word has this feature - but I haven't seen it demonstrated effectively in a web-based discussion forum.

Interesting idea but I cannot advise you how you can go about achieving the result you require.