Hi,
You can do this with Search/Replace very easily.
Put the insertion mark into a Heading3 paragraph. Right-click and select "Copy Special" and then "Paragraph Format".
Then open the Find/Change pod.
In the Find list select Paragraph Tag. Enter "Heading2".
In the Change list select "By Pasting".
Activate "Whole Word" and possibly other "Consider Case". Otherwise you would also find "heading2" or "Heading2AfterHeading" paragraphs.
Then click Find and then Change.
However, be careful!
o Make a backup copy of your file first!
o Make a few test changes first, before you click "Change All".
o Test the file thoroughly after your change!
Best regards, Winfried