Yesterday , I was trying to open the properties of my Windows Phone 7 Project and by mistake ,i converted it to Windows Phone 7.1 .
With few minutes of work , i reverted it back to the Windows Phone 7 version .
I followed the below steps to which worked for me .
1. Unload the project ( 7.1 ) . Right click the project file and click Unload Project .
2. We should edit the project file now . Right click the project and click edit .
3. This will Open up the xml based file which contains the TargetFrameworkProfile tag like
Change this to WindowsPhone
4. Right click on the project and select Reload and click yes on the confirmation message that says the file is open .
5. In the WMAppManifest.xml file , change the AppPlatformVersion to 7.0 .
6. Now right click on the project and select properties , you should see the current version is 7.0 ..
Please note that this will not work incase you have used any functions / API’s specific to Windows Phone 7.1 and then convert it to 7.0 , isnce it might give you reasonable error .