I suggest a feature to export a version as text. One of the uses of the text is a changelog.

You will be able to get the text by clicking "Also available in: TXT" link placed in the bottom right of the version page.

If you export https://www.redmine.org/versions/5 as text, you will get the following output:

# Changelog

## 0.7.3

2008-07-06

Security+Bug fix release

* Defect #52: bulgarian language issue
* Defect #1426: User model does not allow person's first name to contain a dot '.'
* Feature #1506: Add title length note and error info when creating project or creating news
* Patch #1218: Patch for relative links to icon images in 'public/help/wiki_syntax.html'
* Patch #1425: Entourage (and some old client) fails to correctly render notification styles
* Patch #1524: back_url redirects inappropriately when blank
* Patch #1527: Add Project Name to Cross-Project Atom Feed