Q: What does a change from Awaiting Reviewer Assignment to Awaiting Reviewer Selection mean?

Actually, most journals use either of these statuses, not both, to indicate that they are currently seeking peer reviewers. However, some journals do seem to use both statuses to indicate a slight difference between the two. Which is like this.

• Awaiting Reviewer Assignment means the journal editor has started reaching out to potential reviewers for your manuscript.
• Awaiting Reviewer Selection, which is the next stage, means that the editor has received responses from multiple peer reviewers and is presently in the process of making a selection among them. For most manuscripts, in the first round of review, usually two peer reviewers are selected.

For further insights, you may go through this related query by another researcher: Why does my manuscript's status keep changing from "awaiting reviewer selection" to "awaiting reviewer assignment"?

So, great. Your manuscript has cleared desk screening and is being sent for peer review – that’s one step/obstacle crossed. :)

Now, while the editor selects reviewers and the reviewers start and complete their reviews, you may wish to spend some time understanding what happens next. For that, you may find these articles useful:

• Behind the scenes: A journal editor's insight into what happens post manuscript submission
• Behind the scenes: How a journal editor reaches a decision on a manuscript after peer review
• Peer review process and editorial decision making at journals

Hope all that helps. All the best for the next stages on your manuscript!