Assuming that your page numbers reside in either a page header or footer, what allows this is a Word Section Break -- not the same as a page break. It is a feature allowing one to change characteristics of entire sections of a document, such as page margins, headers and footers, and their content. To clarify, a new Word document has one section; as a result, margins, header text or images, etc.. will be the same for all pages.

Breaks are found in the Layout tab of the Ribbon, in the first group. Insert a NEW PAGE section break after the material you wish to be Roman numbered. For example, if that material now ends halfway down page 3, insert the break in an empty paragraph following it. Then, with your cursor on the next page (i.e., the first page of your new Section 2), double-click in the header or footer area, whichever you are using for your numbers. A new tab will appear above the Ribbon, "Header and Footer Tools"; click on it. In the Navigation group, uncheck Link to Previous. This will allow you to set characteristics for the page numbers in your two sections independently of one another.

Assuming you already have, for instance, Arabic numerals (1, 2, 3) for your page numbers, go up and select one in the Section 1, open the Format Page Numbers dialogue from the Design tab on the Ribbon, and select the small Roman numeral Number Format. Pressing the [Esc] key will close the header and footer.