How do I make the case to regain first authorship?
You could make the case that agreements about author order should be renegotiated after a major reworking of the paper. Because this is essentially a new paper, you might convince all parties to change the order. If you regularly correspond with the post doc, you might ask her if she would consider the new author list.

However, if she does not wish to change, you should accept this gracefully. Frankly, you have already missed the appropriate window for renegotiating author order. Author order should be determined before the work (or rework) is done. 
Trying to change the order after work is complete is inappropriate because if, for instance, the third author wanted to keep her place as second, she no longer has any opportunity to do so.

The correct course of action would have been to propose being reinstated as first author before you did the work on the 200 new cases. At this stage, your negotiating power would have been higher. In addition, anyone who wanted to maintain co-first author or second author position could have helped you with the work. It seems you have learned a hard lesson; I hope you can avoid letting it ruin your relationship with your PI.