I have a Word document compiled from a number of shorter Word documents, and in order to keep the section numbering coherent, I need to change all instances of Title style to Heading 1, while keeping the same visual appearance, and then to drop all instances of Heading 1 to Heading 2, H2 to H3, and H3 to H4, likewise preserving the visual appearance.

I have tried what seemed to me the obvious way to do it, which was to select one instance of Heading 3 in the document and apply its style to Heading 4, and then select an instance of Heading 2 and apply the style to Heading 3, and so on. Applying H3's style to H4 worked fine, but when I applied H2's style to H3, that style also got applied to H4. And then H1's style got applied to H2, H3 and H4, until eventually I knew I was in a mess because H1, H2, H3 and H4 all looked like Title.

How can I shuffle the Title and headings styles one step down the hierarchy of headings without getting in such a mess?