This year, Microsoft changed their Dynamics 365 upgrade policies. Some of the changes include delivering two major releases per year (April and October), detailed release notes (you can find these here), and most importantly, enforcing a mandatory update to move all 8.2 users to version 9 by January 31, 2019. You can find more information on the policy changes in this Microsoft blog post and a FAQ on these policies here. Please note that version 8.2 will not be supported after January 31, 2019 and your organization has likely already been contacted by Microsoft to schedule the upgrade.
A related, important reminder for those of you whose organizations are using Field Service or Project Service:
The Project Service Automation solution must be upgraded to version 2.4.4.x or higher to be compatible with Version 9.0
The Field Service solution must be upgraded to version 7.5.1.x or higher to be compatible with version 9.0
I recently went through an upgrade where the client was on Project Service v 1.x and it created a boatload of issues unrelated to the Project Service solution. Upgrading the version to the most recent (2.4.4.x) solved the problems immediately!