After upgrading to a newer preview version it is not possible to revert to a previous version. This is because the preview is not intended to be a full release and as such the previous version is deleted. What kind of error message are you receiving when you try to open one of your other projects?
I installed the latest version of Preview 5 in Unreal Engine 4.7, and now I am unable to access projects that were created in Preview 4. Is there any way I can downgrade to Preview 4?
Something you can try to open the project would be to remove/delete the Intermediate folder and the Saved folder. When you attempt to open the project after this it should force the engine to recompile.