This is not a 2017b1 issue specifically but relates to all versions of Unity and it the reason why I have not installed anything past 5.5 so the feedback might be useful if unwanted.
You seriously need to update the opening project manager to support multiple versions of Unity so that we can run multiple projects with multiple versions of Unity, in the same way that the Unreal launcher works.
Currently I’m a subscriber and my project will be released by the end of July. This project has been ongoing for seven years and I know that if I upgrade the project, it will break. They always do. This is not a problem right now.
However, I will be stating a new project after this and I would like to be able to use the latest version of Unity for that. But I cant, I cant take any risk that the existing project gets upgraded after the game release, so I will be stuck using 5.5 for the foreseeable future, and new projects.
I know you don’t like comparisons to Unreal but you must know that their launcher allows you to have multiple version of Unreal Engine, and it always opens the project with the correct version of Unreal for that project unless you specifically make it upgrade the project to a new version.
This feature is not a ‘nice to have’ but is essential for any small business with multiple projects. Please consider adding this as a matter of urgency.