I have noticed a few instances myself when upgrading a project by creating a copy, that the resulting .uproject file is sometimes still associated with the older Engine version. This results in build failures in Visual Studio similar to what you described.
It should not be necessary to revert back to 4.7 manually, though if you have already edited the .uproject file yourself I would recommend making a copy of your project first. When you select the Switch Engine Version option, it will provide a drop-down list of all installed versions of the Engine. Select the version you wish to use and click OK. This will update the .uproject file to that Engine version, as well as regenerate project files for that Engine version. At that point, the only build errors that should occur would be ones related to changes made in the source code.