You need to add the Administrator-only permission of "Edit-all". This will allow you (if signed in as an Administrator) to edit other people's posts. You could then change the Subject line or body of any message.
That would mean that all future replies made via the forum would default to the new subject line. (If you have any subscribers then those replying via email who still have the original subject line as their default subject.)
Note that when replying anyone can alter the subject line of their own post.
The purpose of the "Change title and meta tags" option is to change the underlying HTML of the page.
The "title" field affects what appears on the tab on your browser and the page title as it appears in search engine results. (i.e. it is the single most important tag on the page for search engine indexing purposes)
These days the "Description" tag is largely redundant. It is text included in the normally invisible headers included in the HTML file that can be part any web page. Search engines may use it to determine if a page is relevant in search results.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.