The answer to this question will depend on the version of Dynamics AX you’re currently on, and the specific reporting and BI tool(s) that you’re using. Microsoft's code upgrade tools for AX2012 R3 customers may upgrade some SSRS reports, and Microsoft is providing a tool to convert BI cubes to the new BI technology. However, the completeness of these upgrades will need to be evaluated on a case-by-base basis.
As a general statement, there will minimally be some work to refactor reports and BI content as you move to Dynamics 365 from any prior version of Dynamics AX.
This will be even more pronounced for clients on older iterations of Dynamics AX 2012 and AX 2009 where the tools and data structure have changed significantly from these versions. Determining the best path for reporting and BI in Dynamics 365 is an important part of the Upgrade Analysis engagement.