Change of author sequence after accept with shepherding
Presumably you know the shepherds and can ask them. But I suspect that if the authors all agree on a change the conference won't object provided that it doesn't come so late as to require changes to websites or proceedings. But in general, order of authorship should be up to the authors. 

I might change this opinion if shepherds had made specific advice about authorship, but I've never seen that done. But still, the paper belongs to its authors.