If you need to run complex find and replace operations using Word's Find & Replace dialogue, you need to clear the parameters before the next operation.
The demonstration below shows you the steps you need to take to restore default search / replace settings in Find & Replace dialogue box.
Open Find & Replace dialogue box (Word 2007 or later: Home tab, Editing group, Find or Replace buttons; Word 2003 or earlier: Edit -> Find or Edit -> Replace menu).
Click 'More' button. Select 'Find what' field; delete text from the field, uncheck 'Use wildcards', click No Formatting button. Select 'Replace with'; delete text from the field, click No Formatting button.
If you have TransTools installed, you can also use Reset Find/Replace dialogue box command to perform the same operation quicker.