How to have the same text in multiple places be changed or refactored at once?

A standard way to handle repeated text is to use bookmarks in conjunction with REF fields. Each bit of repeated text is stored in a bookmark, and each repetition of that text is inserted via a REF field that is linked to the bookmark. So it doesn't matter if two bookmarks happen to store identical looking text at a given time.

1. Select the text that needs to be repeated, and insert a bookmark (on the Insert tab, in the Links group, click Bookmark).
2. At each place where you need the text repeated, insert a REF field (on the Insert tab, in the Text group, click Quick Parts > Field, select Ref, and then select the name of the bookmark in the list).
3. After you change the bookmarked text, select the whole document, and then press F9 to update the REF fields.

As came up in another recent question, it helps if you make bookmarks visible if they aren't (click File > Options > Advanced, and then, under Show document content, select Show bookmarks). Otherwise, when you change the text, some of the new text might end up outside the bookmark, and your changes won't be propagated. When the bookmarks are visible, you can see whether all the new text is correctly inside the bookmark.