Dynamics 365 for Customer Engagement apps: Over the course of April, all environments will be turned “on” to receive the April 2019 features. The updates will be deployed on your environments automatically following our Safe Deployment Process (SDP). If you want to enable the April 2019 features before the update reaches your region, you can enable the updates yourself by opting in to the preview between February 1, 2019, and the date your region will be scheduled to have the features turned on.

However, by the end of April, all instances will have had April ’19 features applied.

Find more at Microsoft docs article: https://docs.microsoft.com/en-us/dynamics365/get-started/release-schedule#frequently-asked-questions