If you need to retain the prior training history, you should consider version control of your content.
When content is revised, the current activity must be changed to "Hide" and a new activity is created with the new content.
Note since enrollment is by a course and not activity the students who have completed the "old content" are still enrolled. If you remove the enrollment the training history is a little challenging to find.
As a best practice, if you need to retain training history, use version control and create a new course for each version change of the content.