Q: What does the status change from "under review" to "in review" mean?

Generally speaking, under review and in review mean the same, i.e. the manuscript is being reviewed by the reviewers. It is difficult to tell why exactly the status of your manuscript changed as you haven’t mentioned any previous status changes. One possible reason for the change could be that “under review” indicates that the reviewer has accepted the invitation to review and “in review” means that the review process has actually started.

However, it is best to write a polite inquiry to the journal editor to find out about the progress of your manuscript.

Related reading:

• How long will the status of my paper show "Under Review?"
• How long does it take for the status to change from "reviewers assigned" to "under review"?