Sounds like you have Automatically Update set for that paragraph style. Try this:
1. Click in an affected paragraph.
2. Click the Home tab on the Ribbon.
3. Click the arrow in the lower right corner of the Styles section.
4. The style for your paragraph should have a bold box around it. You may have to scroll down to find it.
5. Hover the cursor over the style. A listbox arrow should appear. Click it.
6. Select Modify from the list box.
7. Click the Format button and select Paragraph.
8. If the Automatically Update checkbox is checked, uncheck it.
9. Click OK.
10. If you want this to carry forward to other documents based on the template this one is based on, click the radio button for New Documents Based On This Template.
11. OK out of everything.
All paragraphs in the document with that style should adjust themselves.
When you close your document, you may see a warning message asking if you want to save the changes to Normal.dotx. This is an anti-virus measure, intended to alert you that something has changed in Normal.dotx. In this case, you made the change, so you want to accept it.
Now you should be able to change only parts of the document rather than everything.
That should do it.