Acknowledgment works best when it is tied to the job instead of a separate email or training record. When an operator opens a procedure that changed since the last time they ran it, they should see what changed and confirm it before they start, which puts the acknowledgment at the moment it actually matters.
A sign off collected weeks earlier only tells you someone clicked a link, not that they had the change in mind when they did the work. And even an acknowledgment at the right moment is not the same as proof that someone understood the change, which is a separate question about knowledge checks that we cover in another post.