I won't comment about Mandela effect, as I haven't checked yet what it was, nor about these bears I haven't ever heard about, but yes : you can edit your post as long as they're under 7 days old :)

Blockchain will keep tracks of your revisions (you can use https://phist.steemdata.com/history?identifier= + [url of the post or comment you'd want to check])

After 7 days, editing and resteeming are no more accessible.