Document setup top margin change not getting applied

I created a new document in ID with a top margin of .5 inches. 

I imported a Word document that had a top margin of 1 inch and ID correctly formatted the inputted text to a top margin of .5 inches. 

Then I changed the ID margin to 1 inch using File>document setup:

However, ID did not reformat the text in my document. The text stayed at the .5 inch margin. 

Does anyone know why changing the top margin from .5 inches to 1 ich in ID doesn't reformat the text?