Let's say one wants to use remote SQL connection by default and use local SQL connection only when special parameter (called UseLocalSQL) is defined. To implement this sceanrio, first one needs to add the condition to Post build event in VS Project Properties dialog.
if '$(UseLocalSQL)'=='True' Copy "$(ProjectDir)LocalApp.config" "$(TargetPath).config" /y
This means if UseLocalSQL parameter is defined and it's true, the copy .config command will be executed. If local SQL is not needed, default app.config will have remote SQL connection string.
(NOTE: one can also use pre-build event instead of post-build event. It's matter of implementation choice)
If one needs to use MSBUILD tool, the following can be used.
C>msbuild test.sln /t:Rebuild /p:Configuration=Release /p:UseLocalSQL=True
And if one uses TFS build system, the msbuild paramaeter can be specified as follows: