The version of the SNAPSHOT packages is always 2.0.0~SNAPSHOT. I think “apt-get upgrade” only compares the version strings to determine if an upgrade is required. Therefore I would suggest to upgrade SNAPSHOTS installations using the following command:
apt-get --reinstall install openhab2-offline
I think the reinstall behaves like an upgrade if the package is already installed, therefore you don’t have to backup and restore your configuration. Maybe someone can check this.