What is the circumstances will the Editor consider rearrangement of authors after the manuscript has been accepted? We have a medical paper under revision and we hope to be accepted soon. 

In submitted version, last author is head of department and who is the one who had the original idea. He proposed to be first author after acceptance. 

But current first author has done the most work. We are preparing to write a letter to journal. 


Is there any ethical issue in changing author order as I mentioned above?
Under what circumstances will the Editor consider rearrangement of authors after the manuscript has been accepted?
I do not think that the editor will have an issue as long as all other authors (or even corresponding author) have approved the change. To be honest, it is not up to him who to include, omit or lay the order of authors. After all, he is an editor who (most likely) does not know who contributed the most and who did not! To be honest, I have been in a similar situation, and there was not an problem at all. 

Ethically speaking, why does the dept. head want to make this change now (giving that the 1st author has done most of the work)? And why he agreed to be the last author at the first place?  Why is he changing the order after the paper has been accepted? 

I understand that a lot of politics take place in determining authorship order. Unfortunately, in most cases, the person with the most influence tend to get stuff his/her way! But, I do not think that this is ethically right.