Q: What does a change in status from "under review" to "reviewer assigned" mean for a revised manuscript?

The change in status from “under review” to “reviewer assigned” indicates that the assigned reviewer has declined the request to review your paper and the editor had to invite a new reviewer for your manuscript. A revised manuscript is usually sent to the same set of reviewers and it is unlikely that they will decline a review invitation for a revised manuscript unless they are really short of time. This could, therefore, indicate that the paper has been sent to at least one new reviewer.

Sometimes, the editor feels the need for a fresh perspective on a revised paper; hence they might send it out to one new reviewer in addition to the original reviewers. Whatever the reason, the editor will have to look for a new reviewer for your manuscript which is why the status has changed to “reviewer assigned.” 

Related reading:

• Why does the status change back to "Reviewers assigned" after 3 days of "Under review"?
• Why has my article's status changed to "Reviewers Assigned" after two days of "Under Review"?