I tried this using Visual Studio 2008 with Qt Vs add-in 1.1.2 and multiple Qt versions installed.

Create a visual studio project using:

qmake -tp vc

Open Visual studio and try to change the Qt version for this project. This will not work, it is still linking the libraries for the version first set.

Then try to convert this project to an add-in project for Visual Studio. Still can not change the Qt version of the project.

Close the project and delete the vcproj file. Now use the "Open Qt Project File" option from VS to create the vcproj file for your Qt project.
In this project it is possible to change the Qt version.