Q: Is it possible to change the authorship sequence at the proofreading stage?

The proofreading stage is indeed a bit late to change the order of the authors, but it can still be done if the reasons are genuine. Perhaps this author believes that the level of contribution of the various authors changed while making the revisions or that the order was not as per the contribution to begin with. For doing this, you will need to inform the journal editor at the earliest and provide them the written consent of all the authors on the revised order.

Related reading:

• Deciding the order of authors on a paper
• What are the acceptable reasons for changing the order of authors?
• Can I change the order of the first two authors in an article with shared authorship?