Hello Team,
We have used Bloomreach develop edition in 14.5.0, now planning to migrate 15.3 in our system.
Kindly share the document (or) videos for 15.3 version migration and share key points of migration activity its very useful…
@rajamk You need a developer account. Then you can access this Upgrade to Bloomreach Experience Manager 15 - Bloomreach Experience Manager (PaaS/Self-Hosted) - The Fast and Flexible Headless CMS
The important step is upgrade Java from version 8 to 11, update dependencies including 3rd party libraries.
And remember to upgrade from 14.X version to 15.0 version first. Then upgrade from 15.0 to 15.X version.
You can inbox me for more details 
@Quang_Tran Thanks for the prompt response.
Kindly share the 15.0 and 15.3 git repository link. planning to configure in 15.0 in local environment and test the features.