Applying style for headings on a word doc based upon numbering

Is the numbering typed or auto numbering? If the latter, are the numbers part of a single multilevel list? If so, you can use Define New Multilevel List to link the numbering levels to the heading styles.

There are several ways to do what you ask, but the way depends on the document and what you want.

If the numbers are static text and you want them to remain so and if the document headings do not have any headings styles applied, you can use find and replace --from the bottom of the file headings up--to search for the heading numbering--from the lowest number level to the highest.  For example, if 1.1.1.1. is your lowest number level, set up the search by typing ^#.^#.^#.^#. in the  search box.  To set up the replace, leave the box empty but with the cursor in the box click Format > Style and choose your style (such as Heading 4).  This is a regular search that will work even when the next character after the search is inconsistent.  If the next character is consistently a tab or a space, you can add it to the search string and  not worry about searching fro the end of the document.  For more complex differences, use wildcard find and replace.

If  each number level already has a unique (for that document) heading style applied to it and you want to change that style to another one, go to each current heading level in the styles pane.  Select all instances of the style, then apply the new style to the selection.  Or use find and replace here too.

If you want to change the static text numbers to autonumbering, first make sure heading levels are properly applied to each number level (see my first suggestion), then, with the cursor in the first number level 1 heading in the document, apply built-in the multilevel list that is closest to what you want (probably 1 Heading 1, 1.1 Heading 2, etc.).  If necessary, you can change the heading pararaph styles that are linked to the heading levels in the dialogs for  Define new multilevel list (or since you have 100 docs to change and make consistent), Define new list style.  I usually leave the static numbers in place until I'm sure the numbers agree. 

I do most of these things just this way for the "one-offs" I get. Once you decide what you need to do to your documents, you might want to look into writing or obtaining macros to speed things up further.

To add to what Suzanne and Pam have already suggested, if text paragraphs use auto numbering (but have no styles associated with them), you can click to select the list and then apply a heading number format. Some manual "tweaking" may be necessary afterwards, but it should get you started.