I think it is useful to maintain some consistency so I've coded this to always do the partner paragraph so that it is minus then plus. This variation will look to see whether the paragraph contains a minus OR a plus and acts accordingly
Code:
Sub DupParaTwoWay()
Dim aRngPara As Range, aRng As Range, iEqual As Integer, iPlus As Integer, iMinus As Integer
Set aRngPara = Selection.Paragraphs(1).Range
iEqual = InStr(aRngPara.Text, "=")
iPlus = InStr(aRngPara.Text, "+")
iMinus = InStr(aRngPara.Text, "-")
If iEqual > 0 Then
Set aRng = ActiveDocument.Range(aRngPara.Start, aRngPara.Start + iEqual)
If iPlus > 0 Then
aRngPara.InsertParagraphBefore
aRngPara.Collapse Direction:=wdCollapseStart
aRngPara.FormattedText = aRng.FormattedText
If Len(aRngPara.Paragraphs(1).Range.Text) = 1 Then aRngPara.Paragraphs(1).Range.Delete
aRngPara.Characters(iPlus).Text = "-"
ElseIf iMinus > 0 Then
aRngPara.InsertParagraphAfter
aRngPara.Collapse Direction:=wdCollapseEnd
aRngPara.MoveEnd Unit:=wdCharacter, Count:=-1
aRngPara.FormattedText = aRng.FormattedText
aRngPara.Characters(iMinus).Text = "+"
End If
aRngPara.Select
End If
End Sub