Applies to all Dynamics 365 online applications, version 09/18/2018

Microsoft will deliver two major updates per year, in April and October, offering new capabilities and functionality. These updates will be backward compatible, so your apps and customizations will continue to work post-update…. read more

Please review the updated support policies for Dynamics 365 Customer Engagement applications (Sales, Customer Service, Field Service, Project Service Automation, and Marketing) and Finance and Operations.

Important point for Dynamics 365 Customer Engagement application:

• Current version 9.x: You can and have to schedule the major update. If you don’t set a time within the window, Microsoft will schedule an automatic update for your organization and communicate the date.
• Older versions: For Dynamics 365 (online) Customer Engagement applications, we sent update communications in May to all customers running version 8.1 and have scheduled updates. Customers running version 8.2 should plan to update to the latest version by January 31, 2019. If you do not schedule an update to occur on or before January 31, 2019, Microsoft will schedule an automatic update for your organization and communicate the dates. From February 1, 2019, onwards, we will only support the latest generally available version.
• Starting with the April 2019 update, you can validate major updates in a sandbox environment in advance of the update release. This will help you assess productivity and efficiency gains of the new capabilities, as well as train your end users about new capabilities. We will publish the timeline of when to expect updates in sandbox environments in advance. Read more…

Read more in Microsoft docs article: FAQ for Dynamics 365 Update Policies #dynamics-365-customer-engagement-applications