Old project should always be able to open with a newer version. Auto update is relatively new, there is no way to archive older versions of Spine. What you can do is click the cancel button when Spine is downloading a new update (you might need to be fast if you have a super fast connection). This will cancel the download and Spine will run with whatever version you had. You could also block Spine from the internet. But it's really best if everyone is running the latest version.
