I used to be able to redefine styles for the current document just by making a local formatting change, reapplying the style, and specifying that I wanted Word to update the style definition rather than reapply the existing definition. By default, that changed the style definition only in the current document; if I wanted the change to extend to all documents, I had to specifically say so by responding to a prompt asking whether I wanted the changes made to apply to all Word documents. Now it seems to do the latter by default, which is insane.
I don't think that what you've described is actually happening, although the complex relationship between styles in documents and their templates, and the thorny issue of which styles get updated when, can make this appear to be happening. Let's take a look at how Word updates styles and how you can use styles most efficiently in your documents.
Update styles manually rather than "by example"
You can update styles in Word in two ways: "by example" (i.e., by changing the formatting of an instance of the style, and then reapplying the style, usually from the Style drop-down list), or by working in the Modify Style dialog box. Updating styles by example tends to be easier, because you can see the effects of the changes you make, but using the Modify Style dialog box is far less ambiguous, because you can specify whether to add the style change to the template and you can see whether the "Update automatically" box is checked or unchecked for the style. For this reason, it's best to always use the Modify Style dialog box to change styles.
If you do update a style by example and if the "Update automatically" box is unchecked for the style, Word displays a different Modify Style dialog box (see Figure 4-11) to let you decide whether to update the style or reapply it as it stands. You can also choose whether to automatically update the style from now on.