As mentioned NewClass is not meant for change class since Page-New method always create a new page. If a page with same name exists it still creates a new page with this new class and deletes the old page. So it's not changing the class, it's actually deleting the old page.
1) Point 1 is not suggested and should be used with caution. See the notes below related to point 1.
Caution: Except in rare cases, identify in the Step Page field the name of the page to be created by this method. If the Step Page field of the step is blank, this method clears the primary page of the current activity and may change its class. This often is undesirable.
1) If you give both step page and NewClass. A new page will be created with class as NewClass and anything specified in Pages&Classes for that step page will be ignored.
In the above diagram within the page-new previous developers have used DCR functionaliy. What is happening in that case? They have given step page and used DCR also.both the classes mentioned in DCR and step page are also same.