Updating an Old Post
Comments after the post has moved on to greener pastures
Just got done viewing (and commenting on) This Post. I thought I would share what I've learned.
While comments may have been turned off after the payout window closed when @sykochica wrote the post it seems that now you can comment on a post well after its monetary life has ended for the original author.
On a Related Note
It seems that while you may post a comment to an old post (maybe an update to a changed status like the referenced cant make comments feature) you don't seem to be able to edit your old comments or posts once the window is closed. Which brings up a scenario that I think might be useful especially for those posts about the changing status of our wonderful Steemit interface to the block chain.
An Author Can't Change Outdated Information
But, they can go back and provide an update to the people who might find their post months or years later. This could be used either to provide a quick update to the status of the information or, in order to link to a new version if the info has change dramatically.
Can You Bring New Comments to the Top?
If you upvote your new comment on an old post and that changes its status to the most upvoted comment it will jump to the top. If it doesn't, there doesn't seem to be a way to promote the comment like you would a new post. I guess you could pay a service like @randowhale to upvote your post so that it pops to the top????