Yes, that is the screen.
Unless it says remove (Tags), it is an automatic replace. Since the original selection should ONLY be those to be changed.
If you want to ADD a second Author, you must put BOTH in the box. Remember: It is a REPLACE.
Remember to tic the AUTOMATICALLY SET AUTHOR SORT.
You can use this by itself to get rid of those ugly RED Author sort boxes (even with a mix of Authors AS LONG AS to other data fields are used (Tags are allowed)
Tags has an ADD box, and a REMOVE box. you can Add and delete at the same time