possible to share when a post is updated to fb?

The basic assumption is continually posting modifications to an existing public post would be a bug, not a feature.

You author a new post, realize you made a grammar mistake, change it’s to its, and republish. Your audience might be confused to see the same content appear in their newsfeed and your comments on Facebook would split between multiple stories.

In code there is a test from a post changing from a non-public state to a public state and the Facebook identifier of the previously published version of the post preventing multiple publishes of the post.