The emergence of a new version gives you an opportunity of taking advantage of new features and functionalities implemented in this version
Before you can start your migration planning, you may need to answer two important questions: when should you migrate, and how much will it cost in terms of money and effort.
Basically before upgrading a RavenDB to a new version, ensure that the server version is stable. It is not recommended to attempt an upgrade to a new unstable version.
Also, it is not recommended to upgrade to a stable new version if your actual server is undergoing a period of intense change.
Upgrading a large real-world application may take several weeks. During this time, if users are also making changes to current databases, then you have to either make changes in both databases or re-synchronize the databases at a later time by upgrading the changed database again.