I'm sorry, I thought the screenshots were enough

The first screenshot is my document. I want Roman Numeral headings up until 'Introduction', exactly the same way as the second screenshot. Then Numbered headings after this.

With that in mind, you will only be applying the change to that individual style (Heading 1) and you would get a pop-up window reminding you that all remaining headings of that type will also be modified.

So, the data I provided above should work without modifying the remaining styles (Heading 2,3,4...).

If I am in error, I will gladly stand corrected by our resident experts.