Currently, it is not possible to change a body of a historical version of a page or a blog post.
https://developer.atlassian.com/cloud/confluence/rest/api-group-content/#api-api-content-id-put
Error code 409 will be returned.

Why it is important:
1. As a company, we would like to find and clean sensitive content in our Cloud products.
2. Using REST API and some search patterns, we've found that some page has not allowed content (let's say some SSN or email or customer name), not in a current version, but in one of historical versions.
3. We would like to change this particular historical (old) version of a page.
4. When we try to use PUT /wiki/rest/api/content/{id} (https://developer.atlassian.com/cloud/confluence/rest/api-group-content/#api-api-content-id-put) "409 error" returned.

We would like to suggest allowing space administrators to use "PUT /wiki/rest/api/content/{id}" REST endpoint to change content of old versions the same way as current versions.