您所说的错误我认为实际上是"警告"。 - 如下所示？  请注意，此示例中的"错误"列为空，但会显示警告。   如果这是您所看到的，那么一切都应该没问题。 升级后项目是否按预期运行
或者您是否遇到其他问题？

I have created a new light switch project on Visual studio 2012 and when I try to add a new html client it asks me to upgrade. When I upgrade the project is unloaded and tells me the following error:

Studio needs to make non-functional changes to this project in order to enable the project to open in this version and Visual Studio 2010 SP1 without impacting project behavior.

The error your are talking about I think is actually "warning" - as seen below?   Notice the "errors" column is empty in this example but there is a warning.   If this is what you saw then everything should be ok.  Did the project work as expected after the upgrade or did you encounter additional issues?