Scott,
You can do this with an EQ field.
Type: EQ \x\to(Your Word Here) select the entire statement and press
CTRL+F9 then right click and select Update Field.
It you do this routinely, you ming find the following macro useful:
Public Sub Negate()
Dim Expr As String
Expr = InputBox("Enter the expression to negate:", "Negate")
If Expr <> "" Then
ActiveDocument.Fields.Add Range:=Selection.Range, _
Type:=wdFieldEmpty, _
Text:="EQ \x\to(" & Expr & ")", _
PreserveFormatting:=False
End If
End Sub