What is the purpose of updating old content?
For updating errors, the editor should do that on his own based on feedback.
For an addendum, this requires no editing of the original article, but perhaps just a styled box at the top with a few sentences explaining the issue (i.e. this technique is outdated, click here instead for a better method…)
The necessity for an addendum like this would likely also come through comments sent to the editor. If the editor is unskilled to deal with the issue, he could contact the original author, or query the current authorship to write the addendum. I personally don’t think this should cost anything, if a skilled author understands the issue, they could write a paragraph in no time. Perhaps this gives them some extra kudos in the publishing department? More points with the editors?
If an article needs a complete rewrite, like if some tool is drastically updated and Sitepoint wants a fresh look, this sounds like a full cost situation. The original article is little more than a template, as all facts, workflows, and details would have to be tested over again, new features explained, new screeshots taken, etc etc. The offer could go to the original author, then passed on to other authors if not accepted.
Using GIT on Markdown documents to accept pull requests for edits is a decent idea, but requires a lot of work vetting authors, controlling access, dealing with pull requests, setting up the final draft etc. I believe this should be in a private network instead, using a tool that allows for control of users, which documents they can edit, workflows with permission system and version control. Something more like document-based intranet service.
Trello is not without its problems, I don’t believe it’s the best for managing things. It’s hard to deal with revisions, consistent format of documents, attachments, live previews, approval process, etc. It doesn’t allow an easy way to have the peer review system you’d like, or dealing with future revisions and requests.
I would definitely look into an intranet system that allows for revisions, document management, approval/review process with comments, requests/task system, open discussion, user control and permissions system, and maybe even gamification to help analyze star players and engagement.
It would have an attachment system built-in with revisions even on files. A better idea board with discussions and system for accepting an article and moving into an author’s own draft pile. Being able to tag and define and make certain editors the admins over certain topics.
All of this stuff is what an intranet is for. I don’t think Trello is advanced enough, and most other tools cost for every seat. Think in the universe of Huddle, Yammer, Clinked, Basecamp, Podio, Mangoapps, and a dozen more.
However, Trello has its advantages too, that you can have different groups (PHP, Web, etc) but also have different sections within each group. Most intranets have groups and projects, but not an additional layers of sections within those groups for “In Progress” and “Ready for Editor” etc. These workflows and approval process would work different.
I’m sure Sitepoint will figure it out though!