FIRST SCENARIO: You open a document from long ago or far away only to discover there are hard returns (¶) at the end of lines within paragraphs and two or more ¶ between paragraphs. Time for global search and destroy.
Begin by replacing that Times New Roman font, if you like. Do this first so that when you replace manually formatted headings with styles, you won't overwrite the style font.
This one is easy. CTRL+A to select all text. Select your preferred font for body text. You could use the body text style, but that will make it more difficult to identify headings.
1. Temporarily replace double ¶ with a different symbol. Find and replace ¶¶ with @@@. In the find field, type "^p^p" (without quotes). In replace field, type "@@@". Replace all.
2. Find and replace single ¶ with spaces. In the find field, type "^p" and in the replace field, type a space. Replace all.
3. Now you can restore the ¶ you need. Find and replace "@@@" with "^p". Replace all.
4. CTRL+A to select entire document. CTRL+0 to add a single line of space before paragraphs.
Now it's time to replace manual heading formatting with styles.
1. Click in a heading to see what font size is used. If there are also subheadings, check out the size for those also.
2. Open find and replace dialog box and click the MORE button.
3. Click the FORMAT button, then the FONT... option. Enter the font size you want to replace with Heading 1 style. Click OK.
4. Click the replace field to activate it, then the FORMAT button. Select the STYLE... option and select the style you want to use for top-level headings (probably H1). Replace all.
5. Now repeat step 3 finding the font size for subheadings, and step 4 using the H2 style (or whatever you want for subheadings). Replace all.
You can repeat the search and destroy operation, using unique font attributes to identify text that should be formatted with styles.
If you want to replace that manually formatted body text with the body text style...
1. Find the font and size of the body text.
2. Replace with Body Text style. Replace all OR use the REPLACE and FIND NEXT buttons for a more surgical approach.
IMPORTANT! If the original document contained numbered/bulleted paragraphs, you might not want them changed to the Body Text style and the surgical approach, although it might take a bit longer, would probably work best.
If you have several documents to clean up, record these steps as a macro and make a button for your Quick Access Toolbar!