is there a way that we can change the viewers in the post event to feed once it

AFAIK there is no official and supported way to do that.

AFAIK , when I was trying this.... this is how I Did, for the Question what you have asked, the answer WRT me is NO, as Once the Viewers are set you cannot change the viewers however, you can repost the same system feed with updating the security of the viewers using different components. Not sure of any shared components though.

@siddharths To the best of my knowledge, there isn't a direct way to do it.

As a work around, you might create new feed entries with contents and comments exactly same as the old feeds and update the database entities in case if the feed entry ids are being stored. Two problems I can see with this approach is, comments and documents. I am not aware of how to get the document objects associated with a feed entry but you can query the primary database(using Query Database) in order to get the comments(if they are strictly needed) for a given news feed entry. So this way you can design a utility process which creates a new feed entry and its corresponding comments upon the data we have got from making the query to primary database for a given feed entry. At the end you may delete the old feed entries manually. In general, query against Primary Database isn't recommended at all so I would leave it with others to comment on its validity atleast for one time usage.

Thanks for the inputs! For now, we are just going to delete the older news feeds and not going to recreate the same.