You need to insert a section break (Page Layout/Breaks/Next Page) at teh end of the text on the last page of Roman numerals. Then, open your headers/footers on the next page and place your cursor in the footer. Select the Headers and Footers tab (above the others in the ribbon). In the Navigation section, you will see Link to Previous highlighted. Click that and answer the pop-up that you wish to disconnect the footer from the previous section. Check you page numbering to what you want with the page number dialogue. Now the change will no longer affect the previous section.