Tools to Swap Equations in Code

ReSharper 4.5 supports this; select the lines, hit Alt+Enter, and choose Reverse assignments.

Found this too: another example, and explanation on how to install it:

http://www.switchonthecode.com/tutorials/how-to-configure-and-use-visual-studio-macros

You could use Visual Studio Find & Replace to perform the swap. Here's a regular expression pair that will perform the replacement automatically:

Find: ^{:b*}{([^=]+)} += +{([^=]+)};
Replace: \1\3 = \2;

Remember to turn on regular expressions. This will do exactly what you are asking for. This can also be encapsulated into a macro. Here's an example Macro that I put together:

Sub SwapAssignments()
    DTE.Find.Action = vsFindAction.vsFindActionReplaceAll
    DTE.Find.FindWhat = "^{:b*}{([^=]+)} += +{([^=]+)};"
    DTE.Find.ReplaceWith = "\1\3 = \2;"
    DTE.Find.Target = vsFindTarget.vsFindTargetCurrentDocumentFunction
    DTE.Find.MatchCase = False
    DTE.Find.MatchWholeWord = False
    DTE.Find.MatchInHiddenText = True
    DTE.Find.PatternSyntax = vsFindPatternSyntax.vsFindPatternSyntaxRegExpr
    DTE.Find.ResultsLocation = vsFindResultsLocation.vsFindResultsNone
    If (DTE.Find.Execute() = vsFindResult.vsFindResultNotFound) Then
        Throw New System.Exception("vsFindResultNotFound")
    End If
    DTE.Windows.Item("{CF2DDC32-8CAD-11D2-9302-005345000000}").Close()
End Sub

...This will simply swap assignments in the current block.