OK, now that I understand the reasoning...
I believe there's a better, less-risky approach. What I would do is write a completely new post based on information from the old post. At the same time you publish the new post, go back to the old version and add these two things: a canonical tag pointing to the new version, and a bit of _very readable _text at the top linking to the new post. Something like "Hey, thanks for your interest in our content. Feel free to read on, but we thought you should know we've updated this post which can be found here: link"
This accomplishes a few important things. It eliminates the need for a risky project that could affect your entire site just for the ability to update posts (which I'm guessing doesn't happen too often, what percent of posts get updated?). The canonical tag removes the dupe content risk so you're not cannibalizing your own content. And leaving the old post there gives people the opportunity to discover old content that, while possibly not relevant anymore, still demonstrates you've been a trustworthy source of information for a long time.