Reporting and analytics are another area where you can expect major changes when upgrading from Dynamics AX 2012. Dynamics 365 for Finance and Operations introduced modern BI and reporting capabilities. It utilized the SQL Server reporting services for regular document reporting while analytics is powered by Power BI. If you have customized out-of-the-box SSRS reports in AX 2012 - code upgrade does not upgrade these reports; you have to manually do it. 

If you have invested in data warehouse and analytics in AX 2012, you may have to do some work to upgrade this solution to work with Dynamics 365 for Finance and Operations. For example, you may want to utilize the new power BI capabilities provided out-of-the-box or extend this further for your analytical story or use EntityStore to your own data warehouse and build analytics from there. 

Analyze reports that need to be migrated and evaluate the usage of Power BI and other Azure data tools to do away with some reports. You must revisit your data warehouse platform to leverage out-of-the-box provided EntityStore or using aggregated data entities to leverage your own data warehouse.